Remove braces from environment variables in Rose file creation mkdir commands

[chrisbillowsMO]

Closes #201.

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PR creation checklist for the reviewer

• Has <issue_number> above ☝️ been replaced with the issue number?
• Has main been selected as the base branch?
• Does the feature branch name follow the format <issue_number>_<short_description_of_feature>?
• Does the text of the PR title exactly match with the text (not including the issue number) of the issue title?
• Have appropriate reviewers been added to the PR (once it is ready for review)?
• Has the PR been assigned to the developer(s)?
• Have the same labels as on the issue (except for the good first issue label) been added to the PR?
• Has the Climate Model Evaluation Workflow (CMEW) project been added to the PR?
• Has the appropriate milestone been added to the PR?

Definition of Done for the reviewer

• Does the change in this PR address the above issue / have all acceptance criteria been met?
• Does the change in this PR follow the requirements in the wiki: Developer Guide (including copyrights)?
• Have new tests related to the change been added?
• Do all the GitHub workflow checks pass?
• Do all the tests run locally and pass? (Note: the tests are not run by the GitHub workflow, see wiki: Run the tests locally)
• Has the API documentation (e.g. docstrings in Python modules) related to the change been updated appropriately?
• Has the user documentation (i.e. everything in the doc directory) related to the change been updated appropriately, including the Quick Start section?
• Do the HTML pages render correctly? (See wiki: Build the documentation locally)

[Nabil (mo-nabilbrice)]

Thanks for getting this done! All the tests pass and the rose gui has one less error now!

[Emma Hogan (ehogan)]

Excellent, thanks chrisbillowsMO! 🎉

[Emma Hogan (ehogan)]

Nabil (@mo-nabilbrice) please tick the remaining checkbox! 🥳