Check formatting of the patched Tock repository

[gendx]

We should have a CI pass that checks that our patches on top of Tock are properly formatted. We could run cargo fmt --check on third_party/tock/ after the patches being applied.

One caveat is that we currently have custom rustfmt.toml, which cargo fmt will used even when invoked in a sub-directory. Therefore our formatting rules are not the same as the "vanilla" rules expected by Tock, and we get plenty of warnings unrelated to our patches.