buildinstall: Use metadata if skipped

If the phase is skipped, it could mean that we are doing a debug run and
we don't want to mess up the .treefile by missing arch specific images.
The other alternative is that the phase was really skipped, in which
case there will be no files generated and we already handle that fine.

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

tests/test_buildinstall.py

@@ -31,6 +31,8 @@ class BuildInstallCompose(DummyCompose):
         }
         self.all_variants = self.variants.copy()
         self.has_comps = True
+        self.just_phases = []
+        self.skip_phases = []
 
 
 class TestBuildinstallPhase(PungiTestCase):