Helm chart - Fix image pull secrets for signing-key job

Forgotten in https://github.com/matrix-org/dendrite/pull/3428 - Pull secrets need to be used for signing-key job since it uses Dendrite container as well ### Pull Request Checklist <!-- Please read https://matrix-org.github.io/dendrite/development/contributing before submitting your pull request --> * [x] I have added Go unit tests or [Complement integration tests](https://github.com/matrix-org/complement) for this PR _or_ I have justified why this PR doesn't need tests * [x] Pull request includes a [sign off below using a legally identifiable name](https://matrix-org.github.io/dendrite/development/contributing#sign-off) _or_ I have already signed off privately Signed-off-by: `Rhea Danzey <rdanzey@element.io>`