Fix golang tests broken by `git` URL scheme not supported by GitHub anymore

Job #573466 failed for 7e0b41a70376:

The gitlab-grack repository used in submodule tests has a submodule (for its own tests) referencing git://github.com/shacon. Because it is in a submodule declaration that's not within Heptapod Runner, we cannot simply fix as https. Instead, backporting the upstream GitLab Runner fix should be the way to go.

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