task/install: do not duplicate package names

task/install: do not duplicate package names

Get rid of package names duplication when extra_system_packages provided
for rpm.

For example, when it is requested 'bzip2' and 'perl-Test-Harness' to be
installed as extra system packages, there can be observed 5 times
mentioning of the packages, see the log excerpt:

  2025-04-09T12:30:08.360 INFO:teuthology.task.install.rpm:Installing packages: ceph-radosgw, ceph-test, ceph, ceph-base, cephadm, ceph-immutable-object-cache, ceph-mgr, ceph-mgr-dashboard, ceph-mgr-diskprediction-local, ceph-mgr-rook, ceph-mgr-cephadm, ceph-fuse, ceph-volume, librados-devel, libcephfs2, libcephfs-devel, librados2, librbd1, python3-rados, python3-rgw, python3-cephfs, python3-rbd, rbd-fuse, rbd-mirror, rbd-nbd, bzip2, perl-Test-Harness, bzip2, perl-Test-Harness, bzip2, perl-Test-Harness, bzip2, perl-Test-Harness, bzip2, perl-Test-Harness on remote rpm x86_64

Signed-off-by: Kyr Shatskyy <kyrylo.shatskyy@clyso.com>