fix(main/just): fix recipe shebangs

[TomJo2000]

I don't know enough Rust to even begin to reason about why this patch doesn't work.

If someone else could look into it that would be appreciated.
This is intended to solve the shebang incompatibility I found as part of #22291.
It should be as simple as prepending the @TERMUX_PREFIX@ path to the interpreter.
But I have a headache and apparently negative knowledge of Rust.

[TomJo2000]

This doesn't seem to quite fix the XDG_RUNTIME_DIR problem yet.
But the shebang fix seems to be working already.

• At least the minimal example from
  [Bug]: just uses an incorrect runtime directory causing recipes to fail. #22291 passes now.

bug:
    #!/usr/bin/env sh
    echo "hello world"

XDG_RUNTIME_DIR="$PREFIX/var/run/user"
just bug
# hello world

[TomJo2000]

Nvm it works, my dotfiles were broken.
Just the shebangs that needed fixing.

[TomJo2000]

Already verified it on-device just waiting for the CI.

[TomJo2000]

It'd be appreciated if someone could double check this to make sure I didn't make any obvious mistake.

Either @flipee as the maintainer,
or @truboxl or @licy183 as frequent contributors to rust packages would be my choices off the top of my head.

Anyone else is of course welcome to leave a review or comment.

The source location for the patch is at
https://github.com/casey/just/blob/1.36.0/src/recipe.rs#L358