Feature: Improve CI speed

[t3hmrman]

Context

Right now CI speed for each individual run and also when doing releases is pretty slow.

Proposal

There are a few ways we can improve it:

• Parallelize the builds per version of PG (via cargo pgrx init --pg16=download)
• Explicit caching of the pg downloads to a cached folder (right now it should cache but doesn't seem to)
• Caching for steps that are bundled together

While containers should help the build as they have embedded pre-built sources, it might be worth re-evaluating this as builds seem to still do a lot of fresh compilation.

Alternatives

N/A

Additional Links

N/A

27 minute build for a commit to main

[t3hmrman]

Some of this work has happened as a result of other work -- currently the test run (on PRs and pushes to main) takes about 12 minutes.