windows build script: cache pip

previously pip was caching into the wine directory, which we delete and recreate on every run

contrib/build-wine/build.sh

@@ -13,6 +13,10 @@ echo "Clearing $here/build and $here/dist..."
 rm "$here"/build/* -rf
 rm "$here"/dist/* -rf
 
+mkdir -p /tmp/electrum-build
+mkdir -p /tmp/electrum-build/pip-cache
+export PIP_CACHE_DIR="/tmp/electrum-build/pip-cache"
+
 $here/prepare-wine.sh || exit 1
 
 echo "Resetting modification time in C:\Python..."

contrib/build-wine/prepare-wine.sh

@@ -86,8 +86,6 @@ echo "done"
 
 wine 'wineboot'
 
-mkdir -p /tmp/electrum-build
-
 cd /tmp/electrum-build
 
 # Install Python