install.sh: avoid pointless warning when not using a jail
Do not attempt to change the owner of the project database
passwd and group files to the cgi user if a chroot jail is
not in use.
Since sudo privileges are not expected when a chroot jail is
not in use, if the cgi user differs from the mirror user then
a needless warning will be emitted.
Just use the mirror user in this case to avoid the warning.
The group will still be set correctly in either case (or a
warning will be emitted) and that's what really matters
in order to provide write access.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>