Ignore `gprd-checks` when the deployment is triggered from the coordinator-pipeline

After #1578 (closed) we trigger individual deployment pipelines and we execute baking time within the release-tools pipeline. This is causing duplicated messages to be posted in the monthly issue when authorizing a deployment to production, example:

• Message posted by release-tools pipeline - Triggered by auto_deploy:promote:gprd job on the release tools pipeline.
• Message posted by the deployer pipeline - Triggered by the gprd-checks job on the deployer pipeline

When triggering individual deployment pipelines by release-tools we need to somehow ignore gprd-checks job

To do

• Add a variable to skip gprd-checks on deployer pipeline to skip the job if the variable is set https://ops.gitlab.net/gitlab-com/gl-infra/deployer/-/merge_requests/372
• Send that variable when triggering deployments from the single coordinator pipeline - gitlab-org/release-tools!1437 (merged)
• Test. Results here #1688 (comment 562860454)

Development log

• April 26, 2021 - Implementation on deployer and release tools completed-
• April 27, 2021 - A bug around the environment variable logic was discovered on deployer #1688 (comment 561451414). Fix was already merged https://ops.gitlab.net/gitlab-com/gl-infra/deployer/-/merge_requests/373. Waiting for a new pipeline to test the behavior.
• April 28, 2021 - Testing was successfully completed.