Add CI_ENVIRONMENT_ACTION to predefined environment variables
### Release notes
Previously, it was not possible to expose the environment:action as a variable in a pipeline job. Now this is readily available as predefined CI/CD variables by using `CI_ENVIRONMENT_ACTION`.
https://docs.gitlab.com/ee/ci/variables/predefined_variables.html#variables-reference
### Problem to solve
**Problem Statement:** Environment action is not programmatically available to the CI Job itself.
The environment action feature is extremely helpful to pre-stage environments before concluding with the actual deployment, but would be even more useful if jobs could utilize the value to make on-the-fly decisions.
**For example:** we have a script that only runs in a global before_script when it's a deployment (CI_ENVIRONMENT_NAME != null), but should not run when the deployment is only doing a pre-stage.
### Proposal
Add CI_ENVIRONMENT_ACTION to predefined variables https://docs.gitlab.com/ee/ci/variables/predefined_variables.html#variables-reference
## Links to related issues and merge requests / references
issue