Fix issue where keychain github credentials wouldn't work

[tomjonandy]

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This fixes #3363

I would also suggest that lines 32-52 of Library/Homebrew/utils/github.rb be rewritten to use :environment and :keychain rather than re-reading the environment variable (as this would have avoided some of the confusion that this bug caused), although I could be missing something that would mean this wouldn't work. (I would be happy to either add that change to this this pull request or create a new one for the purpose as you see fit).

[MikeMcQuaid]

If I'm understanding this correctly: this will always prefer the environment variable over the keychain if both are set, right? That seems reasonable to me but I'm just checking it is what I think it is.

[tomjonandy]

That was always the case anyway (see the if block in function api_credentials, lines 54 - 71 of Library/Homebrew/utils/github.rb), this fixes a bug where if there was just one thing returned from api_credentials (meaning it is a personal access token, see line 57 of the same file) api_credentials_type was set as :keychain. If it returned 2 things (meaning it is a username and password) it was set as :environent, when those tokens should have been the other way around. This meant that if someone isn't using a personal access token, but has git credentials in keychain, it was treating the password from those credentials as a personal access token, which obviously doesn't work.

[MikeMcQuaid]

Thanks for your great first contribution to Homebrew, @tomjonandy! Without people like you submitting PRs we couldn't run this project. You rock!