utils/github: Say "macOS keychain" in `MissingAuthenticationError`
utils/github: Say "macOS keychain" in `MissingAuthenticationError`

Co-authored-by: Mike McQuaid <a17fed27eaa842282862ff7c1b9c8395a26ac320@mikemcquaid.com>
nandub/brew,nandub/brew,claui/brew,MikeMcQuaid/brew,sjackman/homebrew,EricFromCanada/brew,claui/brew,JCount/brew,vitorgalvao/brew,Homebrew/brew,MikeMcQuaid/brew,Homebrew/brew,EricFromCanada/brew,vitorgalvao/brew,claui/brew,EricFromCanada/brew,vitorgalvao/brew,sjackman/homebrew,JCount/brew,Homebrew/brew,nandub/brew,sjackman/homebrew,nandub/brew,JCount/brew,Homebrew/brew,vitorgalvao/brew,EricFromCanada/brew,claui/brew,sjackman/homebrew,JCount/brew,MikeMcQuaid/brew,MikeMcQuaid/brew