make tests for i3lock and betterlockscreen POSIX compliant

[thatjames]

The following check

if [[ -x '/usr/bin/betterlockscreen' ]]; then
  betterlockscreen -l
elif [[ -x '/usr/bin/i3lock' ]]; then
  i3lock
fi

is not portable. For example, this check fails on nixos due to the way nix symlinks binaries.

command is the POSIX compliant manner to test for binaries. This fixes the issue I have with my nixos build, and should make it easier for newcomers to work with.