Binary Authorization: pass variables needed to create attestations(sign docker images) to gitlab-runner

As part of #7268 we need to have some gcp credentials inside runner in order to create attestations.

See #7268 for details

In this issue we need to:

1. Find a way obtain this credentials(we may already have them stored inside cluster information)
2. Store them if we don't have them already
3. Add predefined variables to runner containing these credentials

cc @ogolowinski @darbyfrey