Send Slack notification if test-install.yml fails

[danielhollas]

This PR adds the Slack notification step to the test-install.yml workflow that runs automatically on the main branch, but until now any failures were not reported, so we for example missed the problem with the new click version (#6885).

• To prevent duplication I've created a new slack-notification action, and use it in both nightly.yml and test-install.yml workflows.

• Lastly, I decided to skip the notifications on runs triggered from pull requests (regardless whether they come from fork or origin) since in that case the failure is obvious in the GitHub UI so no need to send message to Slack.

This PR from is intentionally from the origin repo so that I could test that the notifications actually work, and indeed they are, see messages in the #aiida-core-dev Slack channel from May 26. To see that the notifications are properly skipped, see #6896.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.59%. Comparing base (e1d55fa) to head (d14fa55).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6897   +/-   ##
=======================================
  Coverage   78.59%   78.59%           
=======================================
  Files         567      567           
  Lines       43096    43096           
=======================================
  Hits        33868    33868           
  Misses       9228     9228           

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[agoscinski]

Thanks for the PR. Seems to me not relevant for release so I put it aside to v2.7.1 since the slack devops is a bit complicated and I dont have any experience with

[danielhollas]

@agoscinski no worries. Just btw that I basically just copy pasted the snippet from the nightly workflow.