`command -v` portability

Instead of using the 'which' program to resolve a command to an on-disk
executable filename, use the 'env' program to run it. The former is
non-portable and may not be installed on all systems, while the latter
is guaranteed to be anywhere.

I believe the intention of this code was to ensure that when running
completely arbitrary third-party Exec= specifications, we do not
accidentally end up running a conflicting shell function definition.
Using 'env' fulfills this goal by entering a new environment context
with an unmodified environment (we do not make use of env's stated
purpose of creating a modified enviroment context) and then invoking the
command using exec(3) rather than a shell.