ci: switch cache job to use a specific version of ubuntu

[b31ngd3v]

closes #2820

ubuntu 20.4 seems more suitable

src: https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions

[codecov-commenter]

Codecov Report

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The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2823   +/-   ##
=======================================
  Coverage        ?   82.94%           
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  Files           ?      651           
  Lines           ?    10247           
  Branches        ?     1379           
=======================================
  Hits            ?     8499           
  Misses          ?     1394           
  Partials        ?      354           

Flag	Coverage Δ
longtests	75.90% <ø> (?)
win-longtests	80.60% <ø> (?)

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[terriko]

Thanks! Let's see if that helps or not.