Fix github.ref check for concurrency settings (backport #384)

[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 #384 done by Mergify.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.63%. Comparing base (0b7ed74) to head (48267cc).

Additional details and impacted files

@@           Coverage Diff           @@
##           humble     #385   +/-   ##
=======================================
  Coverage   72.63%   72.63%           
=======================================
  Files          10       10           
  Lines         581      581           
  Branches      223      223           
=======================================
  Hits          422      422           
- Misses         96      105    +9     
+ Partials       63       54    -9     

Flag	Coverage Δ
unittests	72.63% <ø> (ø)

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see 1 file with indirect coverage changes

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