include "build type" along with build number when displaying builds

[bcoe]

We should include the build type and human readable form of matrix values, along with the build # when displaying builds.

See GitHub Actions for inspiration:

I expect this will require both API and frontend work. @StuartRucker has been taking on a lot of backend work lately, so might be good to start splitting this work up amongst ourselves, as we've been doing on frontend work.