Support reading go version from file with setup-go-version-file

[manicminer]

Using the go-version-file argument added in actions/setup-go#62 (comment)

Please note I have not yet tested this end-to-end, I am assuming the desired behavior will occur when setup-go-version is empty and opening this PR in case someone else wants to test it first.

If it works, I'll add a block to the README too.

[bflad]

Hi @manicminer 👋 Thank you for raising this. I verified that actions/setup-go's JavaScript should do the "right" thing here where it will:

• Use go-version if non-empty
• Use go-version-file if go-version is not non-empty
• Generate a warning if both go-version and go-version-file are non-empty, but will prefer the go-version value in that case (seems like a reasonable default)

One small wrinkle is what will happen now if both inputs to these reusable actions are now empty (since the setup-go-version input is no longer required). It should fallback to whatever setup-go's behavior is in that case, but it might not be clear that the lack of a go executable in later steps is failing because of this particular reusable action inputs issue. I think that behavior is acceptable for now and any potential improvements on how to handle this setup-go input issue should be handled upstream.

If you have the opportunity, it would be great to update the README, otherwise we can handle that post-merge when we intend to release this first thing in the new year.

Reference: https://github.com/actions/setup-go/blob/17106403fa5e2516d58cbf6bf1314893d4351d78/src/main.ts#L124-L148