Per conversation with :kmoir, I'm going through untriaged bugs in her components and marking the ones which look to be enhancements/tasks with the enhancement severity to get them out of the triage queue.

If this incorrect, please remove the tag.

I think this bug would be better served by splitting it into two parts:

1. Install sccache in mach bootstrap, which should be straightforward. We produce toolchain binaries for Linux/Windows and I've filed bug 1532329 for producing mac toolchain binaries. I would advise against using cargo install sccache because sccache takes a while to build and some of the build requirements can be annoying.
2. Configure icecream--first, I think long-term we'd be better served by focusing on sccache-dist, as we have a project to install IT-managed sccache-dist schedulers in offices (bug 1524533) currently in progress. We're hoping to get everyone switched over from icecream to sccache-dist in the near future. Second, it would be great if building in an office with a distributed sccache system available could automatically use it, which I think means that the build system would need to do this work at build time instead of at bootstrap time. There are probably some incremental steps we could make along the way here. I did open an sccache issue to give it a way to expose distributed compilation status towards this end.