Migrate from `rtx` to `mise`

[bachya]

Describe what the PR does:

Due to this wildly breaking change: https://mise.jdx.dev/rtx.html#coming-from-rtx

Does this fix a specific issue?

N/A

Checklist:

• [ ] Confirm that one or more new tests are written for the new functionality.
• [x] Run tests and ensure everything passes (with 100% test coverage).
• [ ] Update README.md with any new documentation.

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