Remove compiler from stdenv

[zombiezen]

This Bluesky post suggested removing the compiler as an implicit "stdenv" dependency so that folks have to explicitly declare a dependency on the compiler used. This makes a good deal of sense to me after having been in the guts of bootstrapping for a while, and since we're greenfield, it would be better to do that sooner rather than later.

[zombiezen]

I took a look at this task this morning. This isn't a huge lift, but I think we want to have a compiled version of GCC or clang for macOS before doing this. (#14 or similar.) Otherwise, we are requesting that every developer add __buildSystemDeps on macOS builds, which feels unfortunate.