Send Slack notification if test-install.yml fails - test from fork

[danielhollas]

• Add slack-notification action
• use it in test-install.yml

This is just a test PR, not meant to be merged. The actual PR is #6897.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.59%. Comparing base (e1d55fa) to head (a69d954).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6896      +/-   ##
==========================================
+ Coverage   78.59%   78.59%   +0.01%     
==========================================
  Files         567      567              
  Lines       43096    43096              
==========================================
+ Hits        33868    33869       +1     
+ Misses       9228     9227       -1     

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