Make sure the returned value of an executed sh is still a RunningCommand object.

Debian has transitioned to python3.sh 2.x which now returns a true string
breaking our tools. See:
https://github.com/amoffat/sh/blob/develop/MIGRATION.md#return-value-now-a-true-string



Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>