Avoid warning from pytest

[berland]

PytestWarning: Value of environment variable ERT_STORAGE_ENS_PATH type should be str, but got PosixPath('/.../storage') (type: PosixPath); converted to str implicitly

Issue
⬆️

Approach
str()

• 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 'pytest tests/ert/unit_tests -n logical -m "not integration_test"')

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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.71%. Comparing base (855c517) to head (a107f0e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9633      +/-   ##
==========================================
- Coverage   91.73%   91.71%   -0.02%     
==========================================
  Files         430      430              
  Lines       26639    26639              
==========================================
- Hits        24437    24433       -4     
- Misses       2202     2206       +4     

Flag	Coverage Δ
cli-tests	39.46% <ø> (ø)
everest-models-test	34.19% <ø> (ø)
gui-tests	71.83% <ø> (ø)
integration-test	38.31% <ø> (+0.01%)	⬆️
performance-tests	51.68% <ø> (ø)
test	39.62% <ø> (+<0.01%)	⬆️
unit-tests	73.98% <ø> (-0.02%)	⬇️

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[codspeed-hq]

CodSpeed Performance Report

Merging #9633 will not alter performance

_{Comparing berland:avoid_pytest_str_warning (a107f0e) with main (855c517)}

Summary

✅ 24 untouched benchmarks

[andreas-el]

🌮