Use Bazel cache on Jenkins

[celskeggs]

Our current system of rebuilding everything on every CI cycle makes it take a very long time to learn if a commit is ready to merge. We should consider sharing the Bazel cache between builds in Jenkins so that this is faster.

The primary downside is that flaws in our build scripts or in Bazel could mean this stops being a valid test of whether Homeworld is hermetically buildable. As such, we would probably want to set up a nightly or weekly build that ignores the Bazel cache, and emails us on error, so that we would at least discover this in a reasonable amount of time.