Speeding up Phoenix (Elixir) GitHub Action CI

Part of heyAngus, I’ve put together a pretty speedy GitHub Action CI.

With the right dependency & build caching setup, builds take about a min in total from the previous 4 mins.

(mind you the suite right now runs in just 12s)

Notables

• Works with Elixir 1.13.2, otp: 24.1.7
• Dependency fetching is done in a single standalone job
• Splits out formatting & credo checks
• Splits out the actual mix test run
• Avoids using Postgresql DB checks to shave off 20s+