Pass values between jobs steps

Proposal

Enable users to set a script-level environment variable that can then be passed to other stages in the pipeline.

For example:

• A script could include something like as follows:

pass_values_test_job::
  stage: test
  script:
    - echo "my_expanded_arg=${VARIABLE:12:5} >> $GITLAB_ENV"

• The artifact/cache paths would then be able to use the path /my/path/$my_expanded_arg.