ci: Improve test-msrv job cache

[tottoto]

Improves the test-msrv job cache.

[jplatte]

Want to do sth. like tower-rs/tower-http#635 as well? Then we don't need a nightly toolchain anymore.

[tottoto]

The test-msrv job currently checks both the MSRV build and the minimal dependencies versions build. If it is changed to check only the MSRV build using that approach, a separate job will be needed to verify the build with the minimal dependencies versions, and that job will still require nightly, which is the same approach to the hyper project.

[jplatte]

Right, I guess testing min-versions as well is kinda nice. Though the amount of workarounds needed there is a bit ridiculous. How do you feel about testing msrv in the way I proposed, plus min-direct-versions? I'm pretty sure there's a separate nightly-only flag for that.

[tottoto]

I think it's reasonable. I'll open a pull request to separate these checks.