first-run: Improve install command for Arch.

It seems that partial upgrades (as had been used for first run support
in kdesrc-build) is not supported in Arch [1]. Adding the -u flag helps
with dependency issues during install.

Thanks to nhiga for the report and the patch.

[1]: https://wiki.archlinux.org/index.php/System_maintenance#Partial_upgrades_are_unsupported

Differential Revision: https://phabricator.kde.org/D23173