ci: Use nix-github-actions

GHA is running out of disk, let's run in parallell.

.github/workflows/nix-github-actions.yml

@@ -0,0 +1,31 @@
+name: Nix Flake actions
+
+on:
+  pull_request:
+  push:
+
+jobs:
+  nix-matrix:
+    runs-on: ubuntu-latest
+    outputs:
+      matrix: ${{ steps.set-matrix.outputs.matrix }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: cachix/install-nix-action@v30
+      - id: set-matrix
+        name: Generate Nix Matrix
+        run: |
+          set -Eeu
+          matrix="$(nix eval --json '.#githubActions.matrix')"
+          echo "matrix=$matrix" >> "$GITHUB_OUTPUT"
+
+  nix-build:
+    name: ${{ matrix.name }} (${{ matrix.system }})
+    needs: nix-matrix
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix: ${{fromJSON(needs.nix-matrix.outputs.matrix)}}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: cachix/install-nix-action@v30
+      - run: nix build -L '.#${{ matrix.attr }}'

flake.nix

@@ -12,6 +12,9 @@
 
     zephyr.url = "github:zephyrproject-rtos/zephyr/v3.7.0";
     zephyr.flake = false;
+
+    nix-github-actions.url = "github:nix-community/nix-github-actions";
+    nix-github-actions.inputs.nixpkgs.follows = "nixpkgs";
   };
 
   outputs =
@@ -21,6 +24,7 @@
       zephyr,
       pyproject-nix,
       nixpkgs-python,
+      nix-github-actions,
     }:
     (
       let
@@ -42,6 +46,10 @@
       {
         checks = self.packages;
 
+        githubActions = nix-github-actions.lib.mkGithubMatrix {
+          checks = nixpkgs.lib.getAttrs [ "x86_64-linux" "aarch64-darwin" ] self.checks;
+        };
+
         packages = forAllSystems (
           system:
           let