CI should be caching cargo dependencies

[phated]

Problem

It seems that all cargo dependencies are being rebuilt on every CI run. These should be cached and rehydrated to significantly bring down the CI run times.

Proposed solution

In talking with @kobyhallx, he suggested that maybe this is due to not using "clean sources" within Nix. I believe he has experience with this and will dive in.

Alternatives considered

No response

Additional context

No response

Submission Checklist

• Once I hit submit, I will assign this issue to the Project Board with the appropriate tags.

[kobyhallx]

Spent a few hours on this but src filtering does not seem to be an issue. In buildDepsOnly stage, the actual src is replaced with a dummy one to avoid source changes triggering re-builds.

[phated]

This dependency caching was working on my branch, but seems to still be building deps on master. I think this needs further investigation.