It seems like the solution to this would be to have the build system reproducible from a base image with everything other than the code repo required to successfully kick off a build other than the code repo itself. Uploaded (or made available to upload yourself) to any of the cloud providers (or run locally in a VM), it would allow you to sync the repo and kick off a build for any supported architecture.

Dev not available to integrate a pull request and start a build? Download the appropriate build arch image, fire up VirtualBox, sync the repo (and apply a pull request if the dev hasn't had a chance to do that yet) and start the build script.

This doesn't entirely solve the problem, if nobody has submitted a fix yet, and you don't know enough to pull in the upstream fix and merge it yourself, you're at the mercy of some other user having that knowledge and making a pull request. It does close the gap somewhat though.

Are there existing projects to help get to this level of build reproducibility that can serve as a base to use? It would be awesome to know people are already working on making this easy to adopt.