ci: work-around for broken links in github runners

.github/workflows/build-macos.yml

@@ -122,6 +122,12 @@ jobs:
     steps:
       - name: Update Homebrew
         run: brew update
+      - name: Fix Python symlinks
+        run: |
+          # Github runners have broken symlinks, so relink
+          # see: https://github.com/actions/setup-python/issues/577
+          brew unlink python
+          brew link --overwrite python
       - name: Install LLVM
         run: |
           HOMEBREW_NO_AUTO_UPDATE=1 brew install llvm@${{ matrix.version }}