[9.20] fix: test: Avoid rndc loadkeys race in checkds system test

[9.20] fix: test: Avoid rndc loadkeys race in checkds system test

Address a `checkds` test race condition.

Example failure:

```
________________ test_checkds[incomplete.yes.dspublish.ns2-4-5] ________________
[gw3] linux -- Python 3.13.5 /usr/bin/python3
/builds/isc-projects/bind9/bin/tests/system/checkds/tests_checkds.py:451: in test_checkds
    keystate_check(ns2, params.zone, params.expected_parent_state)
/builds/isc-projects/bind9/bin/tests/system/checkds/tests_checkds.py:183: in keystate_check
    assert val == 0
E   AssertionError: assert '20260528103555 (Thu May 28 10:35:55 2026)' == 0
```

https://gitlab.isc.org/isc-projects/bind9/-/jobs/7497509

Backport of MR !12138

Merge branch 'backport-nicki/fix-checkds-loadkeys-race-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!12150