Fix select-string

[eivindjahren]

Run where this worked correctly on mac: https://github.com/equinor/ert/actions/runs/13025635665/job/36334142343?pr=9895

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'just rapid-tests')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[codspeed-hq]

CodSpeed Performance Report

Merging #9895 will not alter performance

_{Comparing eivindjahren:fix_select_string (181769a) with main (6a0336a)}

Summary

✅ 25 untouched benchmarks

[jonathan-eq]

Very good! Lets make rapid tests rapid again 🥇