My main motivation for this change is to limit the amount of compile
jobs to make sure my machine is still usable for something else when
building a fresh Lix locally.
Also made `build` a dependency of `install`: this is analogous to
`make install` in CppNix where this both recompiles changed files and
installs the artifacts into `outputs/out`. May be a little more pleasant
to work with that, especially when you're used to contributing to
CppNix.
Change-Id: I321e2b0daf1c5e20f82c04e2dd158056c80ed86c
