Loki: Pin pymbolic version to 2022.2 due to upstream incompatibility

[mlange05]

Upstream update to Pymbolic v2024.1 is causing issues in ecWAM regression suite that need separate investigation. Pinning to known-good version with this.

[github-actions]

Documentation for this branch can be viewed at https://sites.ecmwf.int/docs/loki/434/index.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.14%. Comparing base (b12c834) to head (812bd76).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #434   +/-   ##
=======================================
  Coverage   93.13%   93.14%           
=======================================
  Files         200      200           
  Lines       39702    39702           
=======================================
+ Hits        36978    36979    +1     
+ Misses       2724     2723    -1     

Flag	Coverage Δ
lint_rules	96.39% <ø> (ø)
loki	93.09% <ø> (+<0.01%)	⬆️

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[reuterbal]

Many thanks! Confirmed that this is indeed needed right now and pinning the version restores functionality.