github action: only run in the main repo
github action: only run in the main repo

This prevents the action being run in the fork, which
will generate errors due to missing token.

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