mac build: add workaround for installing pyinstaller with setuptools 61

- the mac build broke with 184e122c36
- upstream fix is at https://github.com/pyinstaller/pyinstaller/pull/6701
  however, we are using an old version of pyinstaller atm, so this workaround is easier

log excerpt of failed build:
```
💬 INFO:  Installing PyInstaller.
Processing ./contrib/osx/.cache/pyinstaller
  Preparing metadata (pyproject.toml) ... error
  error: subprocess-exited-with-error

  × Preparing metadata (pyproject.toml) did not run successfully.
  │ exit code: 1
  ╰─> [42 lines of output]
      Traceback (most recent call last):
        File "/Users/user/wspace/electrum/contrib/osx/build-venv/lib/python3.9/site-packages/pip/_vendor/pep517/in_process/_in_process.py", line 363, in <module>
          main()
        File "/Users/user/wspace/electrum/contrib/osx/build-venv/lib/python3.9/site-packages/pip/_vendor/pep517/in_process/_in_process.py", line 345, in main
          json_out['return_val'] = hook(**hook_input['kwargs'])
        File "/Users/user/wspace/electrum/contrib/osx/build-venv/lib/python3.9/site-packages/pip/_vendor/pep517/in_process/_in_process.py", line 164, in prepare_metadata_for_build_wheel
          return hook(metadata_directory, config_settings)
        File "/Users/user/wspace/electrum/contrib/osx/build-venv/lib/python3.9/site-packages/setuptools/build_meta.py", line 188, in prepare_metadata_for_build_wheel
          self.run_setup()
        File "/Users/user/wspace/electrum/contrib/osx/build-venv/lib/python3.9/site-packages/setuptools/build_meta.py", line 281, in run_setup
          super(_BuildMetaLegacyBackend,
        File "/Users/user/wspace/electrum/contrib/osx/build-venv/lib/python3.9/site-packages/setuptools/build_meta.py", line 174, in run_setup
          exec(compile(code, __file__, 'exec'), locals())
        File "setup.py", line 75, in <module>
          setup(
        File "/Users/user/wspace/electrum/contrib/osx/build-venv/lib/python3.9/site-packages/setuptools/__init__.py", line 87, in setup
          return distutils.core.setup(**attrs)
        File "/Users/user/wspace/electrum/contrib/osx/build-venv/lib/python3.9/site-packages/setuptools/_distutils/core.py", line 122, in setup
          dist.parse_config_files()
        File "/Users/user/wspace/electrum/contrib/osx/build-venv/lib/python3.9/site-packages/setuptools/dist.py", line 850, in parse_config_files
          setupcfg.parse_configuration(
        File "/Users/user/wspace/electrum/contrib/osx/build-venv/lib/python3.9/site-packages/setuptools/config/setupcfg.py", line 167, in parse_configuration
          meta.parse()
        File "/Users/user/wspace/electrum/contrib/osx/build-venv/lib/python3.9/site-packages/setuptools/config/setupcfg.py", line 446, in parse
          section_parser_method(section_options)
        File "/Users/user/wspace/electrum/contrib/osx/build-venv/lib/python3.9/site-packages/setuptools/config/setupcfg.py", line 417, in parse_section
          self[name] = value
        File "/Users/user/wspace/electrum/contrib/osx/build-venv/lib/python3.9/site-packages/setuptools/config/setupcfg.py", line 238, in __setitem__
          value = parser(value)
        File "/Users/user/wspace/electrum/contrib/osx/build-venv/lib/python3.9/site-packages/setuptools/config/setupcfg.py", line 552, in _parse_version
          return expand.version(self._parse_attr(value, self.package_dir, self.root_dir))
        File "/Users/user/wspace/electrum/contrib/osx/build-venv/lib/python3.9/site-packages/setuptools/config/setupcfg.py", line 372, in _parse_attr
          return expand.read_attr(attr_desc, package_dir, root_dir)
        File "/Users/user/wspace/electrum/contrib/osx/build-venv/lib/python3.9/site-packages/setuptools/config/expand.py", line 194, in read_attr
          module = _load_spec(spec, module_name)
        File "/Users/user/wspace/electrum/contrib/osx/build-venv/lib/python3.9/site-packages/setuptools/config/expand.py", line 214, in _load_spec
          spec.loader.exec_module(module)  # type: ignore
        File "<frozen importlib._bootstrap_external>", line 850, in exec_module
        File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
        File "/Users/user/wspace/electrum/contrib/osx/.cache/pyinstaller/PyInstaller.py", line 16, in <module>
          from PyInstaller.__main__ import run
      ModuleNotFoundError: No module named 'PyInstaller.__main__'; 'PyInstaller' is not a package
      [end of output]

  note: This error originates from a subprocess, and is likely not a problem with pip.
error: metadata-generation-failed

× Encountered error while generating package metadata.
╰─> See above for output.

note: This is an issue with the package mentioned above, not pip.
hint: See above for details.
```

contrib/osx/make_osx

@@ -136,6 +136,7 @@ PYINSTALLER_COMMIT="40c9abce2d8de879e414fd377c933dccaab1e156"
     popd
     # sanity check bootloader is there:
     [[ -e "PyInstaller/bootloader/Darwin-64bit/runw" ]] || fail "Could not find runw in target dir!"
+    rm pyinstaller.py  # workaround for https://github.com/pyinstaller/pyinstaller/pull/6701
 ) || fail "PyInstaller build failed"
 info "Installing PyInstaller."
 python3 -m pip install --no-build-isolation --no-dependencies --no-warn-script-location "$CACHEDIR/pyinstaller"