Additional improvements to CI workflow

[jessebraham]

This has pretty drastically reduced the runtime on my fork at least. Some additional re-org in here too, sorry for the noise.

Additional considerations:

• The matrices in the check, check-lib, and msrv jobs are different
  • Should we check all platforms/archs for each of these jobs, or is a limited subset adequate?
  • If the latter, which platforms should be checked? Is x86_64 macOS/Linux/Windows enough for the check-lib and msrv jobs? Something else?

[jessebraham]

Guess we need to update the merge queue settings prior to merging this, was a little confused why it was waiting for jobs which don't exist for a minute there 😅

[SergioGasquez]

Should we check all platforms/archs for each of these jobs, or is a limited subset adequate?

I think we should be good with current tests! If not, we can revert this situation in the future

[SergioGasquez]

LGTM! Thanks