Add use_github_file() and use it in use_github_action*()

[jennybc]

Closes #1407, closes #1483

[jennybc]

@jimhester I haven't wired this up to the GHA functions yet, but that's next. If you have any observations, please share. Or wait 'til this is more done.

[jennybc]

@malcolmbarrett Is this what you had in mind?

[malcolmbarrett]

Looks and works great so far! Presumably, you'll refactor use_github_action() to use use_github_file() internally?

[jennybc]

OK @jimhester this is ready for another look. I'm going to call your attention to a few things.

Once this is merged and, presumably, released, you could go back to using symlinks in r-lib/actions.

[jimhester]

I think if we change all the current references to master to HEAD then this should be good to go.

[jennybc]

@jimhester Yes, I just confirmed that HEAD works as the ref everywhere we discuss above.

But from #1483 I got the impression that maybe you wanted to use v1, the same way we're using that as the ref in the actual workflows. It does have a pleasing self-consistency to it.

[jimhester]

That is right, I think using v1 makes sense, lets do that.

[jennybc]

That is right, I think using v1 makes sense, lets do that.

I think it also makes your mental management of the v1 tag easier, if we aren't pulling workflow yaml from HEAD but then, in that same yaml, pulling actual GHA components from v1.

Resolved: I'm going to make v1 the target ref for anything we're pulling out of r-lib/actions.