github(nix): don't build everything twice

When running the `nix build`, the `buildRustPackage` function -- which builds
the `jj` crates -- calls `cargo build --release` with flags like `HOST_CXX`
set. This is called the `buildPhase`. Then, it runs the `checkPhase`, which
calls `cargo nextest`, in our case. However, it does not set `HOST_CXX`, for
some reason.

The intent of `buildRustPackage` is that the `buildPhase` and `checkPhase`
have their compilation options fully aligned so that they reuse the local cargo
`target/` cache directory, avoiding a full recompilation. However, the lack
of `HOST_CXX` above among others causes a cache miss, and a bunch of cascading
recompilations. The net impact is that we compile all of the codebase once in
`buildPhase`, then again in `checkPhase`, making the Nix CI build 2x slower on
average than the other Linux runners; 2-3 minutes versus 7 minutes, on average.

Instead, re-introduce a 'check' into the Flake directly, which overrides the
`jujustsu` package, but stubs out the `buildPhase`. This means it will only run
`checkPhase`, which is what we want. Then, modify the CI to run `nix flake check`
again, like it used to in the past.

Unfortunately, this doesn't fix the cache miss when running `nix build`
yourself, it recompiles from scratch in both phases still. That needs to be
fixed in the future, but it's tolerable for now.

This reverts most of 71a3045032.

Signed-off-by: Austin Seipp <aseipp@pobox.com>

.github/workflows/nix-linux.yml

@@ -19,4 +19,4 @@ jobs:
           fetch-depth: 0
       - uses: DeterminateSystems/nix-installer-action@cd46bde16ab981b0a7b2dce0574509104543276e
       - uses: DeterminateSystems/magic-nix-cache-action@eeabdb06718ac63a7021c6132129679a8e22d0c7
-      - run: nix build --print-build-logs --show-trace
+      - run: nix flake check -L --show-trace

flake.nix

@@ -110,11 +110,30 @@
         };
         default = self.packages.${system}.jujutsu;
       };
+
       apps.default = {
         type = "app";
         program = "${self.packages.${system}.jujutsu}/bin/jj";
       };
+
       formatter = pkgs.nixpkgs-fmt;
+
+      checks.jujutsu = self.packages.${system}.jujutsu.overrideAttrs ({ ... }: {
+        # FIXME (aseipp): when running `nix flake check`, this will override the
+        # main package, and nerf the build and installation phases. this is
+        # because for some inexplicable reason, the cargo cache gets invalidated
+        # in between buildPhase and checkPhase, causing every nix CI build to be
+        # 2x as long.
+        #
+        # upstream issue: https://github.com/NixOS/nixpkgs/issues/291222
+        buildPhase = "true";
+        installPhase = "touch $out";
+        # NOTE (aseipp): buildRustPackage also, by default, runs `cargo check`
+        # in `--release` mode, which is far slower; the existing CI builds all
+        # use the default `test` profile, so we should too.
+        cargoCheckType = "test";
+      });
+
       devShells.default = pkgs.mkShell {
         buildInputs = with pkgs; [
           # The CI checks against the latest nightly rustfmt, so we should too.