DevOps: Add a notification for nightly workflow on fail

[sphuber]

When the nightly.yml workflow fails it will now send a notification to
the dev-aiida-core channel on the AiiDA Slack.

The specific logic in the conditional is on purpose:

always() && (steps.install.outcome == 'Failure' || steps.tests.outcome == 'Failure')

The reason is that without the always(), the step will never run if
any of the previous steps fail, defeating the purpose. However, this
is not enough, because with just always() the final status of the
workflow will be that of the notification step and so will always be
successful. This is confusing since a failed build will be marked as
successful.

By adding the clause after && the build will still be marked as failed
even if the final notification step runs fine.

[codecov]

Codecov Report

Merging #5605 (8231bc0) into main (e141f97) will increase coverage by 0.01%.
The diff coverage is n/a.

❗ Current head 8231bc0 differs from pull request most recent head 287aa75. Consider uploading reports for the commit 287aa75 to get more accurate results

@@            Coverage Diff             @@
##             main    #5605      +/-   ##
==========================================
+ Coverage   79.69%   79.69%   +0.01%     
==========================================
  Files         532      532              
  Lines       37928    37928              
==========================================
+ Hits        30223    30224       +1     
+ Misses       7705     7704       -1     

Impacted Files	Coverage Δ
aiida/common/utils.py	77.26% <0.00%> (+0.43%)	⬆️

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