Using binary cache on inital build

[danielhep]

Following your instructions on MacOS, I'm running the first nix run .#build and I've encountered some very expensive and long running compilation steps.

❯ nix run .#build
warning: Git tree '/Users/danielhep/git/nixos-config' is dirty
Running build for aarch64-darwin
Starting build...
warning: Git tree '/Users/danielhep/git/nixos-config' is dirty
[1/33/87 built, 0.0 MiB DL] building swift-5.8 (buildPhase): [2534/3532] Building CXX object tools/clang/lib/CodeGen/CMakeFiles/obj.clangCodeGen.dir/CGExprScalar.cpp.o

Shouldn't Nix be using the binary cache instead of recompiling swift from source? It was compiling emacs too but I commented that package out for now to save some time.

I'm not entirely sure how cachix works here but I expected it to provide the binary. Maybe it needs to complete the first setup to begin using cachix? It would be nice to have a binary cache bootstrapped on the first setup to save time so I don't have to wait hours for everything to compile.