explicitly install oncall resources in default namespace (#4675)

# What this PR does

Explicitly install oncall resources in default namespace which helps
running OnCall + Incident via ops-devenv

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## Checklist

- [ ] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] Added the relevant release notes label (see labels prefixed w/
`release:`). These labels dictate how your PR will
    show up in the autogenerated release notes.

Tiltfile

@@ -158,7 +158,7 @@ cmd_button(
 helm_oncall_values = ["./dev/helm-local.yml", "./dev/helm-local.dev.yml"]
 if is_ci:
     helm_oncall_values = helm_oncall_values + ["./.github/helm-ci.yml"]
-yaml = helm("helm/oncall", name=HELM_PREFIX, values=helm_oncall_values, set=twilio_values)
+yaml = helm("helm/oncall", name=HELM_PREFIX, values=helm_oncall_values, set=twilio_values, namespace="default")
 
 k8s_yaml(yaml)