[ci] Use dated ROOT/llvm tag to build Cling 1.3

[aaronj0]

ROOT's development workflow between cling and the LLVM fork are slightly different. We cannot rely on a version of cling that was released at a fixed point in time to reliably work with llvm's cling-llvm20 tag which is a moving target as development will continue after the 1.3 release (1.2 had the same issue, see root-project/cling#556). Based on my understanding, we should probably use the dated tag cling-llvm20-20260119-01. I will bump this based on the cppyy migration status (which is the main usecase of having cppyy-cling jobs on our CI in the first place)

[aaronj0]

@mcbarton I believe these changes won't trigger a rebuild unless the cling cache is deleted.. Can you take a look? Thanks!

[mcbarton]

@aaronj0 I won't have time to review this, as I go on annual leave today. If you want the cache to follow your dated root-llvm-tag, then you'll need to set CLING_HASH in the ci accordingly. You'll also need to update the documentation to use these dated commits. Hope this helps fix the ci. You'll probably want to recheck the tests in this issue as part of this work #808

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.56%. Comparing base (a08033c) to head (5dc3193).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #827   +/-   ##
=======================================
  Coverage   79.56%   79.56%           
=======================================
  Files          11       11           
  Lines        4012     4012           
=======================================
  Hits         3192     3192           
  Misses        820      820           

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[aaronj0]

@aaronj0 I won't have time to review this, as I go on annual leave today. If you want the cache to follow your dated root-llvm-tag, then you'll need to set CLING_HASH in the ci accordingly. You'll also need to update the documentation to use these dated commits. Hope this helps fix the ci. You'll probably want to recheck the tests in this issue as part of this work #808

No worries, thanks alot!

[vgvassilev]

LGTM, if the first and last commits are merged.