try to work around gcc update problems

.github/workflows/nightly-Homebrew-build.yml

@@ -44,6 +44,11 @@ jobs:
         if: github.event_name != 'pull_request'
         run: brew update || true
 
+      - name: unlink installed gcc to allow updating
+        run: |
+          brew unlink gcc@8
+          brew unlink gcc@9
+          
       - name: Install prerequisites
         run: brew install --fetch-HEAD --HEAD --only-dependencies --keep-tmp openblas