Set repodata mtime to SOURCE_DATE_EPOCH

repodata/repomd.xml include timestamps of all the other repodata files.
Even when those files are created reproducibly, they have current
modification time. In general case this is a good thing (ease checking
if repodata cache is up to date). But in case of composing installation
image, it breaks reproducibility.
Avoid this by instructing createrepo to set mtime and revision to
$SOURCE_DATE_EPOCH.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>

pungi/gather.py

@@ -1440,6 +1440,9 @@ class Pungi(PungiBase):
             conf.baseurl = baseurl
         if compress_type:
             conf.compress_type = compress_type
+        if 'SOURCE_DATE_EPOCH' in os.environ:
+            conf.revision = os.environ['SOURCE_DATE_EPOCH']
+            conf.clamp_mtime_to = int(os.environ['SOURCE_DATE_EPOCH'])
         repomatic = createrepo.MetaDataGenerator(conf)
         self.logger.info('Making repodata')
         repomatic.doPkgMetadata()