Consider caching packages in buildomat

[smklein]

Having the equivalent of #498 for Buildomat would significantly improve our CI times.

From a fairly recent PR:

• We spend ~2 minutes downloading crates
• We spend ~5-10 minutes compiling third-party dependencies, before we even start compiling any crates within omicron
• We continue spending time compiling third-party dependencies (I saw this running up to ~20 minutes after we started compilation)

Most PRs by developers aren't changing a significant number of these dependencies -- this seems like a solid ~15 minute chunk of time we could shave off of CI, without much risk.

[jclulow]

I have put some thoughts about what we could add to buildomat to support this in oxidecomputer/buildomat#32.