mkosi: Find hostname command on Arch Linux

exec-specifier.service: Executing: /usr/bin/sh -c 'test mkosi-7d5e81c7b81c42338d060a6b98edd44a = $(hostname)'
/usr/bin/sh: hostname: command not found
/usr/bin/sh: line 0: test: mkosi-7d5e81c7b81c42338d060a6b98edd44a: unary operator expected
Received SIGCHLD from PID 7389 (sh).
Child 7389 (sh) died (code=exited, status=2/INVALIDARGUMENT)

gettext provides the hostname binary, but puts it in
/usr/lib/gettext/hostname, which is not part of the default $PATH. Using
inetutils instead puts the binary in /usr/bin/hostname.