openbsd.yml: Try to reduce time

[oech3]

6 min

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/sort/sort-debug-keys. tests/sort/sort-debug-keys is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/shuf/shuf-reservoir (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/sort/sort-stale-thread-mem (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/env/env-signal-handler is now passing!