Allow overriding environment variable with gitlab-runner exec

Description

I have variables defined inside of my .gitlab-ci.yml which I would like to override when running gitlab-runner exec. Using gitlab-runner exec --env does not work as described in #3996 (comment 144993024)

Proposal

Create a new flag --override-env that will allow me to override any environment variable that is defined from the config file or from the .gitlab-ci.yml for local development purposes.