This fixes the Koji build for Fedora 42+, where on an installed system,
`/sbin` and `/usr/sbin` are symlinks to `/usr/bin`; see:
https://fedoraproject.org/wiki/Changes/Unify_bin_and_sbin
On a build root the symlink does not exist, so attempts in the spec to
manipulate binaries in `%{_sbindir}/` fail as these binaries are
installed to `%{buildroot}/usr/sbin` while the script is looking for
`%{buildroot}/usr/bin`
The default value is set to `/usr/sbin` so not passing `SBINDIR` when
installing results in the current behavior being preserved.
Signed-off-by: Michel Lind <salimma@fedoraproject.org>
