Improve CI

[LVala]

This PR:

• adds caching of deps, _build and dialyzer artifacts,
• updates some of the actions versions (and thus removes warnings),
• consolidates everything into a single GH Workflow job (both of previous jobs required compilation and now it's very fast anyways).

Thanks to this the CI should be much, much faster.

Based on https://fly.io/docs/elixir/advanced-guides/github-actions-elixir-ci-cd/ and https://www.alanvardy.com/post/caching-dialyzer with some tweaks here and there.

[codecov]

Codecov Report

Merging #91 (3afaf2c) into master (49dfba7) will increase coverage by 0.05%.
Report is 1 commits behind head on master.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #91      +/-   ##
==========================================
+ Coverage   88.68%   88.74%   +0.05%     
==========================================
  Files          31       31              
  Lines        1503     1502       -1     
==========================================
  Hits         1333     1333              
+ Misses        170      169       -1     

Files	Coverage Δ
lib/ex_webrtc/media/ivf/writer.ex	89.47% <100.00%> (+4.47%)	⬆️

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