TestAlertmanager/rules-create: flaky or new unit test failure

https://github.com/GoogleCloudPlatform/prometheus-engine/actions/runs/15923580406/job/44915771427?pr=1687
```
go: downloading github.com/golang/snappy v0.0.4
=== RUN TestAlertmanager
main_test.go:78: >>> deploying static resources
main_test.go:93: >>> waiting for operator to be deployed
main_test.go:97: >>> waiting for operator to be ready
main_test.go:101: >>> operator started successfully
=== RUN TestAlertmanager/rules-create
ruler_test.go:407: creating rules
ruler_test.go:440: Internal error occurred: failed calling webhook "validate.globalrules.gmp-operator.gmp-system.monitoring.googleapis.com": failed to call webhook: Post "[https://gmp-operator.gmp-system.svc:443/validate/monitoring.googleapis.com/v1/globalrules?timeout=10s](https://gmp-operator.gmp-system.svc/validate/monitoring.googleapis.com/v1/globalrules?timeout=10s)": tls: failed to verify certificate: x509: certificate signed by unknown authority
```