.gitlab-ci.yml:
Remove legacy keyword 'cobertura' and use the gitlab >= 15 based
`coverage_report` which is used to specify format and path to coverage
XML.
This invokes the dedicated ci keyringctl command that is responsible
to check with sq-keyring-linter and hokey when completely unknown
new keys are added to the keyring.
.gitlab-ci.yml:
Add rule to run `make lint` if `keyringctl` changes in a merge request.
Add integration stage to always attempt to build and install the keyring
in a containerized environment.