ci: Use mise to manage tools and tasks

[abhinav]

Trying out using mise to manage dev dependencies
as well as dev-time tasks like running tests, etc.

Required a small change to the mise bootstrap script based on its location.
The script adds itself to the $PATH if used.

[abhinav]

This change is part of the following stack:

• ci: Use mise to manage tools and tasks #542 ◀

_{Change managed by git-spice.}

[abhinav]

Been experimenting with this, and it works well enough.
Merging this in until/unless there are issues down the line.