Fix relative path not found on production web server

What does this MR do?

Fixes #277405 (closed)

Looks like puma in production does not run from Gitlab root, so relative paths don't work. This is different from the GDK configuration which is why this was not seen during development. https://gitlab.slack.com/archives/CB3LSMEJV/p1604684707464000?thread_ts=1604683110.462900&cid=CB3LSMEJV

We already have specs to verify the file loads properly, but they can't account for this configuration discrepancy: https://gitlab.com/gitlab-org/gitlab/-/blob/master/spec/lib/gitlab/ci/runner_instructions_spec.rb#L78-88

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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