Replace `os_setenv` uses with `os.spawn`

[gucci-on-fleek]

This is more of an FYI than an actual issue/feature request, but os.spawn now takes a second argument that lets you set the environment for the command to use. This is considerably less clunky than the current os_setenv string solution, so at some point in the future if/when you decide to have l3build require TeX Live ≥ 2025 (probably many years from now), you can switch to the new function.

(This was somewhat suggested by @josephwright a couple months ago.)