sources/ostree: capture ostree output

Instead of using stderr for the ostree subprocess command
capture its output so that in the case of an error we get
properly return the error output. With the old behavior
all the `ostree` command output would land in the journal
of the worker.

sources/org.osbuild.ostree

@@ -67,7 +67,8 @@ def ostree(*args, _input=None, **kwargs):
     print("ostree " + " ".join(args), file=sys.stderr)
     subprocess.run(["ostree"] + args,
                    encoding="utf-8",
-                   stdout=sys.stderr,
+                   stdout=subprocess.PIPE,
+                   stderr=subprocess.STDOUT,
                    input=_input,
                    check=True)