lint errors for ci in deployer project

I'm seeing these errors (which I guess are non-fatal) in all deployer pipelines:

https://ops.gitlab.net/gitlab-com/gl-infra/deployer/-/jobs/10929853

$ find .gitlab-ci.yml ci -type f -iname "*.yml" -exec yamllint {} --no-warnings \;
ci/environments/gprd-cny.yml
  75:3      error    duplication of key "variables" in mapping  (key-duplicates)
  215:3     error    duplication of key "variables" in mapping  (key-duplicates)
ci/environments/gprd.yml
  94:3      error    duplication of key "variables" in mapping  (key-duplicates)
  300:3     error    duplication of key "variables" in mapping  (key-duplicates)
ci/environments/gstg-cny.yml
  69:3      error    duplication of key "variables" in mapping  (key-duplicates)
  243:3     error    duplication of key "variables" in mapping  (key-duplicates)

It's probably not doing any harm but we should fix them, also maybe make this yamllint a hard failure once it is resolved?

cc @ahyield @skarbek