workflows/tests: run vendored gems on macOS 11.
workflows/tests: run vendored gems on macOS 11.

All existing workflows using this seem to be broken.

konqui/brew,vitorgalvao/brew,nandub/brew,EricFromCanada/brew,claui/brew,Homebrew/brew,konqui/brew,MikeMcQuaid/brew,JCount/brew,sjackman/homebrew,EricFromCanada/brew,EricFromCanada/brew,MikeMcQuaid/brew,JCount/brew,sjackman/homebrew,nandub/brew,sjackman/homebrew,vitorgalvao/brew,MikeMcQuaid/brew,Homebrew/brew,konqui/brew,JCount/brew,konqui/brew,Homebrew/brew,sjackman/homebrew,EricFromCanada/brew,vitorgalvao/brew,JCount/brew,Homebrew/brew,nandub/brew,vitorgalvao/brew,MikeMcQuaid/brew,nandub/brew,claui/brew,claui/brew,claui/brew