fix: Automatically install nightly toolchain when missing

[byfnoel]

This change ensures that the required Rust toolchain is installed automatically if it is missing.

Fixes: #2051

[byfnoel]

I'd love to get this PR reviewed @GuillaumeGomez @syphar.

[GuillaumeGomez]

Still gonna wait for @syphar to take a look.

[syphar]

thank you for the fix! I remember this annoys me every time I have to set up a fresh environment.

I can do some manual testing later this week,

but until then it would be cool if you could add tests for add-essential-files with or without toolchain?

[byfnoel]

but until then it would be cool if you could add tests for add-essential-files with or without toolchain?

Thank you, @syphar. I added tests that try the new behavior.

[syphar]

thanks!

[syphar]

@byfnoel the tests fail on master, can you check why?

[syphar]

fyi, I reverted the changes on master (#2904) because I wanted to deploy master, and couldn't invest more time into debugging the test failure on master.

Feel free to recreate the PR, with a fixed test or functionality.

[byfnoel]

I will take a look to see why it was failing as soon I have the time to do so over the next few days. @syphar