Consider re-running the sanity check if the external toolchain changes

[kergoth]

I doubt we want to checksum the entirety of the toolchain, but right now the sanity check triggers solely based on the external toolchain path, so if the contents of that path change, it won't re-run the sanity test. At a minimum we could checksum the gcc binary, or just naïvely go based upon the modification timestamp of the directories we care about.