Add CI_COMMIT_SHORT_SHA to gitlab-runner-exec

What does this MR do?

This simply adds a precomputed environment variable containing the first eight characters of CI_COMMIT_SHA.

Why was this MR needed?

It's handy for versioning and other purposes to associate a shorter ID with a build, change, etc. We use it in some cases in our own software:

func (b *AbstractShell) writeCheckoutCmd(w ShellWriter, build *common.Build) {
	w.Notice("Checking out %s as %s...", build.GitInfo.Sha[0:8], build.GitInfo.Ref)
	w.Command("git", "checkout", "-f", "-q", build.GitInfo.Sha)
}

Are there points in the code the reviewer needs to double check?

Don't think this really needs test but open to hearing otherwise.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

Expands on https://gitlab.com/gitlab-org/gitlab-ce/issues/48061