Use fully qualified references to invoke local actions inside `bake_targets.yml`

[NiccoloFei]

bake_targets.yml is a reusable workflow that's also getting called by other CNPG repositories, such as https://github.com/cloudnative-pg/postgis-containers.
In order to reach local postgres-container actions when running the reusable workflow from another repo, we need to reference them using the fully qualified syntax.