Fix github.ref check for concurrency settings (backport #1872)

[mergify]

github.ref:

The ref given is fully-formed, meaning that for branches the format is refs/heads/<branch_name>.

which means the leading slash was wrong, see this canceled job for example.

https://docs.github.com/en/actions/reference/workflows-and-actions/contexts#github-context

---

This is an automatic backport of pull request #1872 done by Mergify.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.09%. Comparing base (41c4dc2) to head (4ac5935).
⚠️ Report is 4 commits behind head on humble.

Additional details and impacted files

@@           Coverage Diff           @@
##           humble    #1873   +/-   ##
=======================================
  Coverage   67.09%   67.09%           
=======================================
  Files         122      122           
  Lines       14548    14548           
  Branches     8988     8988           
=======================================
  Hits         9761     9761           
  Misses       1461     1461           
  Partials     3326     3326           

Flag	Coverage Δ
unittests	67.09% <ø> (ø)

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