stages/rpm: set encoding for subprocess.run()

Printing 'owner' in an fstring triggers the mypy warning 'str-bytes-safe'

  Error: If x = b'abc' then f"{x}" or "{}".format(x) produces "b'abc'",
  not "abc".

We can't yet use the nicer run() argument, 'text=True', because we still
target the RHEL 7 build root which only has Python 3.6 and 'text' was
introduced in Python 3.7.

stages/org.osbuild.rpm

@@ -112,6 +112,7 @@ def remove_unowned_etc_kernel(tree, rpm_args):
                           "--root", tree,
                           "-qf", "/etc/kernel"],
                          stdout=subprocess.PIPE,
+                         encoding="utf-8",
                          check=False)
     owner = res.stdout.strip()