fix(ci): ignore errors brew upgrade

Apparently it is not safe to run on GitHub environment, so avoid failing
on that. We still try the upgrade because sometimes it is necessary.

Author: nijel

.github/workflows/macos.yml

@@ -39,6 +39,7 @@ jobs:
         persist-credentials: false
     - run: brew update
     - run: brew upgrade
+      continue-on-error: true
     - run: brew list --versions
     - run: brew deps --tree --installed
     - run: brew config