cmd-diff: fix superfluous .stdout call

The .stdout at the end of this line is unnecessary and has no effect.
When stdout is redirected to a file object, the stdout attribute of
the returned CompletedProcess object is None.

Assisted-By <gemini-code-assist>

coreos#4253 (comment)

Author: dustymabe

src/cmd-diff

@@ -442,7 +442,7 @@ def diff_cmd_outputs(cmd, path_from, path_to, strategy='template'):
             else:
                 assert strategy == 'cd'
                 workingdir = path
-            subprocess.run(c, cwd=workingdir, check=True, stdout=output).stdout
+            subprocess.run(c, cwd=workingdir, check=True, stdout=output)
         git_diff(from_output.name, to_output.name)