Make direnv optional

[Jacobb164]

Summary

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Checklists

This pull/merge request meets the following requirements:

• [x] code runs
• [ ] developments are ethical and secure
• [x] you have made proportionate checks that the code works correctly
• [x] you have tested the build of the template
• [ ] test suite passes
• [ ] minimum usable documentation written in the docs folder

Comments have been added below around the incomplete checks.