CI_COMMIT_SHORT_SHA is not set

Summary

CI_COMMIT_SHORT_SHA environment variable is not set.

Steps to reproduce

Add env as a script.

.gitlab-ci.yml

debug:
  script:
    - env

Actual behavior

The environment variable is empty.

Expected behavior

Should have the short version of the latest commit sha.

Used GitLab Runner version

Version:      12.2.0
Git revision: a987417a
Git branch:   12-2-stable
GO version:   go1.12.9
Built:        2019-08-23T08:17:01+01:00
OS/Arch:      darwin/amd64