install-dep: Use command -v instead of which

`command -v' is shell builtin required by POSIX [1] and supported on all
common shells (bash, zsh, dash, busybox sh, mksh). `which' utility is not
presented on some containers (e.g. Fedora, openSUSE), also going to be
removed from future Debian versions.

Also remove stderr redirection to /dev/null as it's unnecessary when
using 'command': POSIX says "no output shall be written" if the command
isn't found.

[1] https://pubs.opengroup.org/onlinepubs/9699919799/utilities/command.html
[2] https://salsa.debian.org/debian/debianutils/-/commit/3a8dd10b4502f7bae8fc6973c13ce23fc9da7efb

Signed-off-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Steve Dickson <steved@redhat.com>