CI overhaul with better cache keys and act support

[doughsay]

Multiple changes to the CI script:

• CI can now run locally using act. Caching doesn't work locally (also, one of the cache keys can't be generated locally, see here: hashFiles documentation says you can supply multiple paths, but act doesn't let you nektos/act#324).
• Elixir and OTP versions DRYed into variables.
• Cache keys include Elixir and OTP versions, to help invalidate stale caches when versions change
  • This is done using this: Adds exact installed versions of Elixir and OTP as outputs actions/setup-elixir#36 (waiting on approval and release to update action reference)
• Dialyzer PLTs have been moved into _build and are cached alongside it.
• The steps have been refactored to all be identical between jobs except the one or two steps that are relevant to that job. This prepares us for "composite actions", which is an upcoming feature from github. This will drastically DRY up the CI script.
• The codecov token has been removed, they now support actions officially

[codecov]

Codecov Report

Merging #38 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #38   +/-   ##
=======================================
  Coverage   94.77%   94.77%           
=======================================
  Files          13       13           
  Lines         153      153           
=======================================
  Hits          145      145           
  Misses          8        8           

Impacted Files	Coverage Δ
lib/afk/keycode/transparent.ex	100.00% <ø> (ø)

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