Add new predefined variable CI_COMMIT_SHORT_SHA

What does this MR do?

Adds a new variable to the environment containing the first eight characters of the SHA

What are the relevant issue numbers?

• Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/48061
• Related to gitlab-runner!1116 (closed), which adds the same variable to gitlab-runner exec

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer a