Fix buildfarm failures from 2af07e2f74.

Fix buildfarm failures from 2af07e2f74.

Use GUC_ACTION_SAVE rather than GUC_ACTION_SET, necessary for working
with parallel query.

Now that the call requires more arguments, wrap the call in a new
function to avoid code duplication and offer a place for a comment.

Discussion: https://postgr.es/m/E1rhJpO-0027Wf-9L@gemulon.postgresql.org