Don't use capture_output=True with subprocess.run()

The `capture_output` argument does not exist on Python 3.6 used by
default on RHEL-8.

Signed-off-by: Tomáš Hozza <thozza@redhat.com>

tools/osbuild-mpp

@@ -360,7 +360,7 @@ class ImageManifest:
             src = f"docker://{imagename}"
 
         res = subprocess.run(["skopeo", "inspect", "--raw", src],
-                             capture_output=True,
+                             stdout=subprocess.PIPE,
                              check=True)
         m = ImageManifest(res.stdout)
         m.name = imagename