Allow jobbermaster/jobberrunner to be installed into /usr/lib/jobber

[fennerm]

Installing anything into /usr/libexec or /usr/local/libexec is strongly discouraged on Arch Linux.
https://wiki.archlinux.org/index.php/Arch_package_guidelines

To package jobber on arch, the sytemd service file needs to be changed to read:
ExecStart=/usr/lib/jobber/jobbermaster

Unfortunately it appears that there is an explicit check for jobberrunner in /usr/libexec or /usr/local/libexec which causes jobber v1.4 to fail on Arch even with the correct systemd config:

Apr 13 14:58:24 cmp /usr/lib/jobber/jobbermaster[488]: Failed to launch runner for fen: Failed to find jobberrunner in [/usr/libexec /usr/local/libexec].

Is it possible to also check for jobberrunner in /usr/lib/jobber?