This commit replaces the out-of-date builder in the flake which relied
on submodules for fetching and the compiler for building. Now we
disable fetching and building explicitly with the environment variable
and then use builtins.fetchGit and a derivation mostly derived from
upstream to compile the grammars.
Anecdotally, this is still quite slow as builtins.fetchGit does not
seem to do shallow clones. I'm not sure I see a way around it though
without recording sha256s, which seems cumbersome.
