This adds a new build stage called combine which takes the results
of the previous compile builds and packages them up into one
release tarball per platform.
Historically Vere has used a simple 'vx.y.z' versioning scheme that
predates the monorepo structure introduced in 7ce50ad. Going forward
it's expected that our blessed runtime king/serf binaries will follow
the scheme 'urbit-vx.y.z' to disambiguate them from other repository
subprojects.
This renames Vere release tarballs as their version followed by their
target platform. So with the updated tag scheme, instead of e.g.
'urbit-linux64-vx.y.z.tgz' these will appear along the lines of
'urbit-vx.y.z-linux64.tgz'.
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.
