Use Github actions cache for caching snaphots instead of an external server

[Kobzol]

Hi! This action is really useful, however sadly it seems to me that it no longer works, since the external server used to store the computed snapshots seems to be offline. This would probably happen sooner or later with any server, so I tried to create a PR that completely removes the reliance on any external server.

When a push event occurs, a snapshot is computed and then stored into the github actions cache.
When a pull request event occurs, the reference snapshot is:

1. Restored from cache, if the corresponding cache entry already exists
2. Or computed from scratch, if it doesn't

Even if there is no cache hit, the reference snapshot recomputation will be usually quite fast, because the crate was already pre-built when the PR snapshot was computed, so typically there will not be too many changes that need to be compiled.

The code currently assumes that the workflow uses the checkout action, which is true for most workflows I suppose.

You can see the modified version in action here.

[alerque]

I know some time has past (and @orf I saw the abuse from a user you got a few years back) but I'm happy to report that the code from this PR still works. Notably the existing version(s) are currently broken because of #403 and this caching mechanism fixes that an the action works as expected. I would recommend it for merging and a new release be tagged.

[Kobzol]

Well, this works, but it's not really ideal, security wise. pull_request_target is prone to cache poisoning, and GitHub kind of acts like this issue doesn't exist :/ It's sad that safely writing to cache (or even posting a PR comment) from a PR coming from a fork is so complicated on GitHub.

[Kobzol]

I think that a better solution is to avoid any cache at all, and just compile the baseline commit and the PR commit each time, and then display the difference. This still leaves the problem with posting comments on PRs from forks though. I think that these sorts of actions would be better implemented as GitHub apps.