Remove source install of gtest branch in CI (backport #2284)

[mergify]

This is an automatic backport of pull request #2284 done by Mergify.
Cherry-pick of f9f1804 has failed:

On branch mergify/bp/iron/pr-2284
Your branch is up to date with 'origin/iron'.

You are currently cherry-picking commit f9f180471.
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	both modified:   .github/workflows/ci.yaml

no changes added to commit (use "git add" and/or "git commit -a")

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[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (1955e57) 50.50% compared to head (1955e57) 50.50%.

❗ Current head 1955e57 differs from pull request most recent head edf5581. Consider uploading reports for the commit edf5581 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             iron    #2313   +/-   ##
=======================================
  Coverage   50.50%   50.50%           
=======================================
  Files         386      386           
  Lines       31732    31732           
=======================================
  Hits        16022    16022           
  Misses      15710    15710           

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[henningkayser]

this needs to be merged for CI to succeed