Failing build: release-master: ERROR: Unable to validate cosign version: 'v1.13.1'

[wallrj]

After merging #471 I noticed that the badge on the README file is red and says "release-master: failing"

• https://github.com/jetstack/jetstack-secure/actions/workflows/release-master.yml

Urgent because it's likely to prevent us releasing a new version and is likely to alarm anyone visiting this repo to examine the source of the agent.

/kind bug