github/workflows: unlink macOS pkg-config if installed

This works around an issue with recent Github CI runners.

.github/workflows/build.yml

@@ -233,6 +233,10 @@ jobs:
           find /usr/local/bin -lname '*/Library/Frameworks/Python.framework/*' -delete -print
           brew unlink python && brew link --overwrite python
 
+      - name: Unlink pkg-config if the CI runner happens to have it already installed
+        run: |
+          brew ls --versions pkg-config && brew unlink pkg-config || :
+
       - name: Change Xcode version
         if: ${{ matrix.xcode != '' }}
         run: |