Building Julia for Nix

Which would mean that Miles’s build is doing a source-build of GMP (with our GMP patches applied).

Looks like we only carry two patches for GMP:

  1. julia/deps/patches/gmp-alloc_overflow.patch at master · JuliaLang/julia · GitHub
  2. julia/deps/patches/gmp-exception.patch at master · JuliaLang/julia · GitHub