Fix a few more Zizmor-flagged workflow concerns

[N-Wouda]

This PR:

• Closes #xxxx.
• Adds and passes relevant tests.
• Has well-formatted code and documentation.

To check:

• CI
• CD
• CodSpeed
• DOC

Details

Notes:

Please read our contributing guidelines first.
In particular:

• You must add tests when making code changes.
  This keeps the code coverage level up, and helps ensure the changes work as intended.
• When fixing a bug, you must add a test that would produce the bug in the master branch, and then show that it is fixed with the new code.
• New code additions must be well formatted. Changes should pass the pre-commit workflow, which you can set up locally using pre-commit.
• Docstring additions must render correctly, including escapes and LaTeX.
• Finally, it is essential that all contributions in this PR are license-compatible with PyVRP's MIT license.
  Please check that this PR can be included into PyVRP under the MIT license.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.17%. Comparing base (6d709e1) to head (548f329).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1092   +/-   ##
=======================================
  Coverage   95.17%   95.17%           
=======================================
  Files          77       77           
  Lines        4146     4146           
  Branches     1346     1346           
=======================================
  Hits         3946     3946           
  Misses         18       18           
  Partials      182      182           

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[codspeed-hq]

Merging this PR will not alter performance

✅ 57 untouched benchmarks

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_{Comparing restrict-default-workflow-permissions (548f329) with main (6d709e1)}