When using Hydra to build machine configurations, you'll often see
"nixosConfigurations.foo" five times, i.e. for each build step being
run. This isn't very helpful I think because in such a case, a single
build step can also be compiling the Linux kernel.

This change also fetches the `drvpath` and `type` from the `buildsteps`
relation. We're already joining it, so this doesn't make much difference
(confirmed via query logging that this doesn't cause extra SQL queries).

Unfortunately build steps don't have a human readable name, so I'm
deriving it from the drvpath by stripping away the hash (assuming that
it'll never contain a `-` and that `/nix/store/` is used as prefix). I
decided against using the Nix bindings for that to avoid too much
overhead due to store operations for each build step.

When building e.g. nixpkgs, the "Running builds" view will mostly look
like this

    hello.x86_64-linux (Build of hello-X.Y)
    exa.x86_64-linux (Build of exa-X.Y)
    ...

This doesn't provide any useful information. Showing the step name only
makes sense if it's not a child of the job's derivation. With this
patch, that information will only be shown if the drv name (i.e. w/o
`/nix/store/` prefix, .drv ext & hash) is not equal to the drv name of
the job itself (build.nixname).