Fix CI on Solidus v3.1 by explicitly fetching solidus_frontend from GH

[waiting-for-dev]

Summary

This is the same fix that was used in 9f1f60bf5a4032c3c94cc906b074245f4ab01b97. Otherwise, bundler will try to fetch the gem from RubyGems and complain because not finding v3.1.10.dev version.

Closes #239

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