Backend: CI_DEPLOY_* variables not set if gitlab-deploy-token is defined in group settings
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### Summary
As described [here](https://docs.gitlab.com/ee/user/project/deploy_tokens/index.html#gitlab-deploy-token), you could add a special deploy token, which would be exposed as environment variables in a CI job. Doing so in a project works fine, but I need this deploy token to be valid for all projects in this group. So I added a group deploy token with the same name. Unfortunately after this change the CI_DEPLOY_USER and CI_DEPLOY_PASSWORD variables are undefined in the CI job.
### Steps to reproduce
- Add a group deploy token with name 'gitlab-deploy-token'
- Run a CI pipeline in a project within this group.
### Workaround
1. Created a deploy token in group level named `gitlab-deploy-token`
1. Set group level environment variables `CI_DEPLOY_USER` and `CI_DEPLOY_PASSWORD` to `gitlab-deploy-token` and the value thereof, respectively.
### Proposal
1. Either provide the values automatically
1. or add the workaround in the documentation
### Example Project
Example project: https://gitlab.com/justinvtime-test/test
Build with the project deploy token defined:
https://gitlab.com/justinvtime-test/test/-/jobs/505669965
Build with the group deploy token defined:
https://gitlab.com/justinvtime-test/test/-/jobs/505668102
### What is the current *bug* behavior?
the CI_DEPLOY_USER and CI_DEPLOY_PASSWORD variables are undefined in the CI job
### What is the expected *correct* behavior?
the CI_DEPLOY_USER and CI_DEPLOY_PASSWORD variables are defined in the CI job
issue