Fix setting environment variable in triggered builds

Closes: #2564 (closed) Triggered docker builds from CE/EE are not currently being tagged correctly. All of them get the latest tag and hence overwrite each other.

We were using wrong set of brackets around env variables in Makefile. Instead of fixing it inside the Makefile, I have moved it to .gitlab-ci.yml. This is more consistent as we are originally setting the variable in gitlab-ci.yml - in both CE/EE repo and omnibus repo. So it is easier to understand if it is handled there itself at the receiving end. Also, setting environment variables inside Makefile inside a conditional block gave some problems.

When triggered builds are made a rake task, this can go inside the rake task logic.