This is required to prevent `rm -r ./pier/stage` from crashing the
script when ./pier/stage doesn't exist, and seems to be necessary for
soliding a pill with the associated derivation.
* Add cross-compilation for `lmdb`.
* Got built caching working in CI with `cachix`.
* Cache cross compilation dependencies and toolchains.
* Do release builds in CI.
* Upload release builds to `bootstrap.urbit.org` on successful build.
* Lots of optimization work for CI.
* Boot from a solid pill in CI and load arvo with `-A`.
* Increase `vere` HTTP timeout to 15m.
* Move extraneous stuff out of pkg/urbit/*
* s/urb/herb/g
* Removed some boilerplate for `urbit` builds.
* Build urbit tests and run them in the nix build.
