Add a couple of missed .gitignore entries.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: 8afca702ecbf0730b6175afda0cecbbf0a1967e7
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2020-12-18T21:24:49Z
Releases: 14.0
Add a couple of missed .gitignore entries.

Any subdirectory that's ignoring /output_iso/ should also
ignore /tmp_check_iso/, which could be left behind by a
failed pg_isolation_regress_check run.

I think these have been wrong for awhile, but it doesn't
seem important to fix in back branches.

Files