Caching efficiently in CI

[llebout]

Currently we don't cache anything, and that's very wasteful.

We should follow actions-rs/meta#21 closely for the availability of better caching support in the Github Actions helpers we use.

And we are lucky to have the maintainer of these helpers as a committer already @svartalf!

[svartalf]

It is possible to cache Rust artifacts to some extent, as in this snippet: https://github.com/actions/cache/blob/master/examples.md#rust---cargo. Combined with that pretty valuable information from the https://github.com/actions-rs/meta#21 it can potentially speed up the CI execution right now.

[ghost]

PR: #69

[llebout]

@stjepang Even though that PR can be merged, I suggest we keep this open because it's far from optimal currently. So there's still this question of properly caching things.

[llebout]

See: #69 (comment) for details on the limitations of the current Github Actions's caching mechanism

[ghost]

Closing this issue due to inactivity. I didn't find the lack of caching to be a huge problem :)