Add CI instructions when strictNullChecks fails due to Messenger

[fregante]

What does this PR do?

Instead of having to ask around, I figured I'd encode these instructions into the CI. Similar to:

• https://github.com/pixiebrix/pixiebrix-extension/pull/7746/files

See the hidden message below for an example. It's hidden automatically when the issue is resolved.

Checklist

• Designate a primary reviewer: @grahamlangford

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.15%. Comparing base (bff97ac) to head (37eb9e7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7748   +/-   ##
=======================================
  Coverage   72.15%   72.15%           
=======================================
  Files        1267     1267           
  Lines       39706    39706           
  Branches     7370     7370           
=======================================
  Hits        28650    28650           
  Misses      11056    11056           

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[fregante]

For a real-life example, see #7734 (comment)

I think it's useful because it immediately leads me to what file needs to be changed.

[grahamlangford]

This is an awesome addition. Thanks!

[github-actions]

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