osbuild: replace capture_output in subprocess.run

The `capture_output` argument for subprocess.run was added in 3.7,
but want to support 3.6 as well. Change all the usages of it with
`stdout=subprocess.PIPE` that will have the same effect, at least
for stdout.

test/mod/test_util_ostree.py

@@ -78,7 +78,7 @@ class TestObjectStore(unittest.TestCase):
                      "tree",
                      "--print-only",
                      path],
-                    capture_output=True)
+                    stdout=subprocess.PIPE)
             self.assertEqual(r.returncode, 0)
             js = json.loads(r.stdout)