Fixes missing $USER value in ArchLinux sh (bash)

Co-authored-by: Marc Cornellà <hello@mcornella.com>

Closes #4962

Closes #6555

Co-authored-by: Mike Mattice <mmattice@reliant.io>

Fixes #10587

Fixes #10578

The previous check only worked if the user could run `sudo` without typing the
password, which is almost none (I checked in Google Cloud Shell so I failed to
notice this).

This new check works whether the user has no sudo privileges, or if it has,
whether they have to type in the password or not.

It should really be easier to check if the user doesn't have privilege without
having to make them type the password.

This fixes the error in Google Cloud Shell, where a password prompt
appears when running `chsh` but the user (hello) does not have a
password.

If ran with `sudo`, the `chsh` command happens without a password
prompt.

When generating completions for Cargo, if the Rust sysroot
(i.e. `rustc +${${(z)$(rustup default)}[1]} --print sysroot`) contains
spaces, Cargo completions will not work because the spaces are not
escaped, thus passing two arguments to the "source" command instead of
one. The spaces need to be escaped for this to work.