CI: run all-features workspace tests on ARM Linux

[weiyaoluo (SeliMeli)]

The all-features workspace tests currently run only on x86-64 runners. Add ubuntu-24.04-arm so optional-feature tests and doctests also execute on ARM Linux.

• Descriptive title suitable for release notes
• New dependencies: none
• API changes: none
• Documentation changes: none

Reference Issues/PRs

Separated from #1287.

What does this implement/fix? Briefly explain your changes.

Adds one runner to the existing test-workspace-features matrix. The commands and other jobs are unchanged.

Any other comments?

Validation: parsed the workflow with PyYAML, checked that only the intended matrix entry changed, and passed git diff --check. ARM test execution will run in CI.

[Codecov Comments Bot (codecov-commenter)]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.52%. Comparing base (49cdb88) to head (dc8479c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1430   +/-   ##
=======================================
  Coverage   91.51%   91.52%           
=======================================
  Files         527      527           
  Lines      103055   103055           
=======================================
+ Hits        94309    94318    +9     
+ Misses       8746     8737    -9     

Flag	Coverage Δ
miri	91.52% <ø> (+<0.01%)	⬆️
unittests	91.26% <ø> (+<0.01%)	⬆️

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see 3 files with indirect coverage changes

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