sources/skopeo: override exists() method

The parent class exists() method checks if there is a file in the
sources cache that matches the checksum.  For containers, the item is a
file called container-image.tar under a directory that matches the
checksum, so for containers it always returned False.

sources/org.osbuild.skopeo

@@ -115,6 +115,9 @@ class SkopeoSource(sources.SourceService):
             with ctx.suppress_oserror(errno.ENOTEMPTY, errno.EEXIST):
                 os.rename(archive_dir, f"{self.cache}/{image_id}")
 
+    def exists(self, checksum, desc):
+        return os.path.isfile(f"{self.cache}/{checksum}/container-image.tar")
+
 
 def main():
     service = SkopeoSource.from_args(sys.argv[1:])