Release tarballs previously didn't include a release directory, so the
files would be unceremoniously dumped into the user's current directory
when extracted.
Fixes#1400.
* 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.
