Boilerplate checker does not run in CI

[munnerz]

When CI runs, the boilerplate checker fails to complete (but does not fail the CI pipeline) due to python missing in the test image, with the message:

/usr/bin/env: 'python': No such file or directory

This means we are not ensuring source files have correct boilerplate/copyright headers.

[munnerz]

Example: https://prow.infra.cert-manager.io/view/gs/cert-manager-prow-artifacts/pr-logs/pull/cert-manager_csi-lib/67/pull-cert-manager-csi-lib-verify/1836149614542065664

[munnerz]

We should also include standard bash headers in https://github.com/cert-manager/csi-lib/blob/main/hack/verify-all.sh (around errexit, nounset etc) so this fails CI in future: https://github.com/cert-manager/csi-lib/blob/main/hack/verify-all.sh