backend: enforce createrepo_c gzip compression

Fedora 39+ changed the default compression to ZSTD, but this is not
supported on EPEL7 and older hosts.  We need to implement some better
"if EL7 or older" logic, but this is a quick pre-release fix that will
keep our deployment with GZIP after the F37 => F39 move.

Relates: #3021

backend/run/copr-repo

@@ -137,8 +137,9 @@ def _database_option(chroot: str) -> str:
 
 
 def run_createrepo(opts):
+    compression = "--general-compress-type=gz"
     createrepo_cmd = ['/usr/bin/createrepo_c', opts.directory, _database_option(opts.chroot), '--ignore-lock',
-                      '--local-sqlite', '--cachedir', '/tmp/', '--workers', '8']
+                      '--local-sqlite', '--cachedir', '/tmp/', '--workers', '8', compression]
 
     if "epel-5" in opts.directory or "rhel-5" in opts.directory:
         # this is because rhel-5 doesn't know sha256

backend/tests/test_modifyrepo.py

@@ -428,7 +428,7 @@ def test_copr_repo_run_createrepo(self, popen, do_stat, chroot, database_option)
         assert popen.call_args_list[0][0][0] == \
             ["/usr/bin/createrepo_c", repodir, database_option,
              "--ignore-lock", "--local-sqlite", "--cachedir", "/tmp/",
-             "--workers", "8", "--update"] + additional_args
+             "--workers", "8", "--general-compress-type=gz", "--update"] + additional_args
 
     @pytest.mark.skipif(
         distro.id() == 'fedora' and int(distro.version()) >= 36,