nix: merge (now redundant) flake check with normal build

Summary: Since 066032b6e6 was merged, the `nix flake check` build no longer
overrides the 'cargo test' profile explicitly, to save disk space. The CI seems
to be in a better spot. This will stem the tide for a while hopefully.

However, with that change in place, the `nix flake check` build was
essentially a redundant, nearly-identical copy of a normal `nix build` with no
differentiating features, except: `RUST_BACKTRACE` is set to 1.

Delete all this code, and remove it from the CI matrix, and instead just export
`RUST_BACKTRACE` on the `checkPhase` of the normal `nix build` instead, which is
functionally equivalent.

Also does some minor, no-functional-change touchups to `flake.nix` while I was
there (whitespace, etc.)

Signed-off-by: Austin Seipp <aseipp@pobox.com>
Change-Id: I87336b16e2a0b973343ecbde8ffd7b8f

.github/workflows/nix-linux.yml

@@ -10,12 +10,7 @@ permissions: read-all
 
 jobs:
   nix:
-    strategy:
-      fail-fast: false
-      matrix:
-        os: [ ubuntu-latest ]
-        mode: [ "flake check", "build" ]
-    runs-on: ${{ matrix.os }}
+    runs-on: ubuntu-latest
     name: nix-build
     timeout-minutes: 20
     steps:
@@ -24,4 +19,4 @@ jobs:
           fetch-depth: 0
       - uses: DeterminateSystems/nix-installer-action@07ebb8d2749aa2fac2baae7d1cfa011e4acfd8d1
       - uses: DeterminateSystems/magic-nix-cache-action@a04e6275a6bea232cd04fc6f3cbf20d4cb02a3e1
-      - run: nix ${{ matrix.mode }} --print-build-logs --show-trace
+      - run: nix build --print-build-logs --show-trace

flake.nix

@@ -48,6 +48,7 @@
         jujutsu = ourRustPlatform.buildRustPackage rec {
           pname = "jujutsu";
           version = "unstable-${self.shortRev or "dirty"}";
+
           buildFeatures = [ "packaging" ];
           cargoBuildFlags = ["--bin" "jj"]; # don't build and install the fake editors
           useNextest = true;
@@ -57,9 +58,8 @@
             "^flake\\.lock$"
             "^target/"
           ];
-          cargoLock = {
-            lockFile = ./Cargo.lock;
-          };
+
+          cargoLock.lockFile = ./Cargo.lock;
           nativeBuildInputs = with pkgs; [
             gzip
             installShellFiles
@@ -72,11 +72,14 @@
             darwin.apple_sdk.frameworks.Security
             darwin.apple_sdk.frameworks.SystemConfiguration
             libiconv
-            ];
+          ];
+
           ZSTD_SYS_USE_PKG_CONFIG = "1";
           LIBSSH2_SYS_USE_PKG_CONFIG = "1";
           NIX_JJ_GIT_HASH = self.rev or "";
           CARGO_INCREMENTAL = "0";
+
+          preCheck = "export RUST_BACKTRACE=1";
           postInstall = ''
             $out/bin/jj util mangen > ./jj.1
             installManPage ./jj.1
@@ -93,11 +96,6 @@
         type = "app";
         program = "${self.packages.${system}.jujutsu}/bin/jj";
       };
-      checks.jujutsu = self.packages.${system}.jujutsu.overrideAttrs ({ ... }: {
-        preCheck = ''
-          export RUST_BACKTRACE=1
-        '';
-      });
       formatter = pkgs.nixpkgs-fmt;
       devShells.default = pkgs.mkShell {
         buildInputs = with pkgs; [