Don't bother registering QEMU for amd64 builds

.github/workflows/build.yml

@@ -29,6 +29,7 @@ jobs:
     steps:
       - uses: actions/checkout@v1
       - name: Register Docker QEMU
+        if: matrix.arch != 'amd64'
         run: docker run --rm --privileged multiarch/qemu-user-static --reset -p yes
       - name: Build bitcoind
         run: docker build --build-arg ARCH=${{ matrix.arch }} --build-arg VERSION=${{ matrix.version }} -t bitcoind:v${{ matrix.version }}-${{ matrix.arch }} .