Always copy normalized files after a regress run (#6254)

Our python based tests didn't always copy the normalized files after the
regress run. I had the problem where running the following command would
result in non-normalized files in the expected directory after running
our PG upgrade tests locally:

```
cp src/test/regress/{results,expected}/upgrade_list_citus_objects.out
```

This PR fixes that by always running `copy_modified` even if the tests
fail. The same was already being done for our perl based tests at the
end of the `pg_regress_multi.pl` file.

src/test/regress/citus_tests/common.py

@@ -137,7 +137,9 @@ def create_citus_extension(pg_path, node_ports):
 
 def run_pg_regress(pg_path, pg_srcdir, port, schedule):
     should_exit = True
+    try:
         _run_pg_regress(pg_path, pg_srcdir, port, schedule, should_exit)
+    finally:
         subprocess.run("bin/copy_modified", check=True)