Updates Github Action runners for PGX

[BradyBonnette]

This update includes a few things that help speed up Github Action runs and also give slightly better output for the end-user.

• A proper balance of throughput between number of parallel jobs and execution time per job, while also making it clearer about what job is ran and what has failed
• Uses Github Actions cache + sccache within each runner to speed up subsequent runs when only codebase code has changed
• Includes a MacOS build to ensure that any changes to the codebase will still build on a Mac (macos-11)

[eeeebbbbrrrr]

Just a few comments from our discussion this morning:

• Trim down the cargo pgx init jobs to only do pg14.
• Shorten job titles to "pgx-tests & examples", "cargo pgx init", and "MacOS build & test"

[eeeebbbbrrrr]

Please see my comments on the PR itself. After those are addressed I believe this is good to merge!