Refresh CI?

[BradyBonnette]

After some discussion, we have noted that the plrust CI may need a little love. It's been a while since updates have been performed on it, and now that a bunch of stuff has changed both internally in the plrust project and with some of it's dependencies like PGX, it might be time to refresh it a little.

Some ideas to start off:

• [x] Reintroduce ability to use sccache for aarch64 (broken via using plrustc)
• [x] Use sccache for the x86_64 tasks (and generate the cache keys appropriately, including the pg version in the filename)
• [x] Maybe break the trusted vs. untrusted tests into separate jobs (might see a speedup here, or maybe not!)
• [x] Try to figure out how to re-enable the MacOS builds (they were disabled originally because things kept breaking)
• [x] Enable the use of RUSTFLAGS=-Dwarnings to fail the build if there are any warnings present
• [ ] Possibly gate the build with some quick-running tasks, like rustfmt and audit?
• [ ] Maybe some general formatting/restructuring of ci.yml so that it's easier to find in both the yaml file and the resulting output in the Github Actions pages
• [ ] Make use of pre-built docker images that we can use with Github Actions container registry: https://docs.github.com/en/packages/working-with-a-github-packages-registry/working-with-the-container-registry
• [ ] ???