qemuvariants: Extract ostree repo in mutate image, not class init

A whole big twisted chain here; `cosa buildextend-aws` was crashing
in the FCOS promote job I think becuase it allocates a separate
tempdir.  The data we need (ostree commit) is in the `build_dir`.

So we could likely just tweak that, but while we're here, move
the import into the "prepare" phase and not the class init so
it's more "on demand".

src/cosalib/qemuvariants.py

@@ -184,9 +184,6 @@ def __init__(self, **kwargs):
 
         _Build.__init__(self, **kwargs)
 
-        repo = os.path.join(self._workdir, 'tmp/repo')
-        import_ostree_commit(repo, self._tmpdir, self.meta)
-
     @property
     def image_qemu(self):
         """
@@ -235,6 +232,10 @@ def mutate_image(self):
         :param callback: callback function for extra processing image
         :type callback: function
         """
+
+        repo = os.path.join(self._workdir, 'tmp/repo')
+        import_ostree_commit(repo, self.build_dir, self.meta)
+
         work_img = os.path.join(self._tmpdir,
                                 f"{self.image_name_base}.{self.image_format}")
         final_img = os.path.join(os.path.abspath(self.build_dir),