ci: add s390x big-endian workflow for hash function tests

[ijatinydv]

Hi @jvdp1,

Following up on our discussion in #1130 , this PR adds a dedicated CI job to catch any future Big-Endian regressions.

It uses uraimo/run-on-arch-action to emulate an s390x (IBM Z) environment using QEMU. Since QEMU emulation is notoriously slow, I've heavily optimized the workflow so it doesn't bottleneck the rest of the project's CI or burn through Actions minutes:

• Targeted builds: Instead of building all of stdlib, it uses cmake --build build --target test_hash_functions. This only compiles the hash modules and their direct dependencies, completely skipping heavy modules like linalg or stats.
• Smart triggers: On PRs, this job will only run if src/hash/, test/hash_functions/, or the workflow file itself is modified. Otherwise, it just runs on merges to master.
• Caching & Safety: Uses the GitHub token to cache the apt-get dependency layer, and adds --no-tests=error to ctest to ensure the CI forcefully fails if the test target gets renamed in the future.

I've tested this on my fork and it successfully catches the endianness issues we just fixed. Let me know if you'd like to tweak the scope or triggers at all!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.05%. Comparing base (ee33055) to head (8d27d63).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1132      +/-   ##
==========================================
+ Coverage   68.02%   68.05%   +0.03%     
==========================================
  Files         402      402              
  Lines       12839    12839              
  Branches     1383     1383              
==========================================
+ Hits         8734     8738       +4     
+ Misses       4105     4101       -4     

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