Add support for gotip and latest

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This PR adds support for building Go tip from source and an alias latest for the major release.

Closes #21
Closes #31
Closes #32
Closes #35

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/cc @zikaeroh

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Ubuntu tests are passing, getting errors on Windows.

https://github.com/tie/setup-go/runs/349414830

Edit: seems related https://stackoverflow.com/a/37996249

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Aside from Windows errors, the code is working. Opening for review.

[zikaeroh]

I left some comments, but I'm still pretty iffy on the magic of using an API to fetch the commit, grabbing an archive, and creating a VERSION file, given how far it is from an actual build that people working with tip actually do.

The main use case of running tip in CI is to find issues ahead of time, be it on Go itself or in user code. For example, I found broken unsafe code in one of my dependencies because I ran race tests with tip, which enabled checkptr and revealed the issue.

Being too far removed from the version output expected by the Go devs, or the install process in the documentation I think may not be a good idea. I'd really consider another method where a plain git clone --depth=1 ... is used.

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I’ve reconsidered my decision on using archives.

Plain git downloads around 138M, while the archive size is 21M (compressed) and 113M (extracted). The difference is negligible. Also, using git should fix issues with Windows.

We could cache .git separately and do git pull --depth=1 for updates, which should be overall faster than downloading archive each time.

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Almost there on another branch, having a problem with lint/prettifier being not helpful at all (it just fails and tells to read the log file for details).

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Also closes #35 because I ended up refactoring tests and found this issue.

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Ready for review again.

/cc @zikaeroh I hope I’ve addressed your concerns.

Last CI run: https://github.com/tie/setup-go/commit/8d6c88b10d1e37b1bffe029bd5e441eacf067633/checks

[bryanmacfarlane]

Interested in these features potentially but master changed drastically with the v2-beta changes. Do you want to take a stab at implementing on top of that code?

[bryanmacfarlane]

Looking over this soon. assigning to me

[wyk9787]

Is there an estimated timeline when this will be implemented? This will be really helpful for us to transit into using Github Action.

[tie]

Closing this PR since it’s out-of-date.