Try adding a CI job matrix

.github/workflows/CI.yaml

@@ -3,10 +3,28 @@ on:
   workflow_call:
 
 jobs:
-  check:
+  matrix:
     runs-on: ubuntu-latest
+    outputs:
+      matrix: ${{ steps.set-matrix.outputs.matrix }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: DeterminateSystems/nix-installer-action@main
+      - id: set-matrix
+        name: Generate Nix Matrix
+        run: |
+          set -Eeu
+          matrix="$(nix eval --json '.#githubActions.matrix')"
+          echo "matrix=$matrix" >> "$GITHUB_OUTPUT"
+
+  build:
+    name: ${{ matrix.name }} (${{ matrix.system }})
+    needs: matrix
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix: ${{fromJSON(needs.matrix.outputs.matrix)}}
     steps:
       - uses: actions/checkout@v4
       - uses: DeterminateSystems/nix-installer-action@main
       - uses: DeterminateSystems/magic-nix-cache-action@main
-      - run: nix flake check
+      - run: nix build -L '.#${{ matrix.attr }}'

flake.nix

@@ -3,9 +3,16 @@
 
   inputs = {
     flake-parts.url = "github:hercules-ci/flake-parts";
+    nix-github-actions = {
+      inputs.nixpkgs.follows = "nixpkgs";
+      url = "github:nix-community/nix-github-actions";
+    };
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
     systems.url = "github:nix-systems/default";
-    treefmt-nix.url = "github:numtide/treefmt-nix";
+    treefmt-nix = {
+      inputs.nixpkgs.follows = "nixpkgs";
+      url = "github:numtide/treefmt-nix";
+    };
   };
 
   outputs =
@@ -16,6 +23,7 @@
         ./nix/dev-shell.nix
         ./nix/lint.nix
         ./nix/web.nix
+        ./nix/ci.nix
       ];
     };
 }

nix/ci.nix

@@ -0,0 +1,12 @@
+{
+  inputs,
+  self,
+  lib,
+  ...
+}:
+{
+  flake.githubActions = inputs.nix-github-actions.lib.mkGithubMatrix {
+    # To save time, we only test on x86 Linux in CI.
+    checks = lib.getAttrs [ "x86_64-linux" ] self.checks;
+  };
+}