fix(workflow): checkout before local action and remove nested checkout

Add top-level `actions/checkout` to jobs so local action files are available, and remove redundant checkout from `install-nix` composite action. Prewarm flake devShell remains in the action.

.github/actions/install-nix/action.yml

@@ -3,8 +3,16 @@ description: Composite action to checkout the repo and install Nix with flakes e
 runs:
   using: composite
   steps:
-    - name: Checkout
+    - name: Install Nix
-      uses: actions/checkout@v5
+      uses: cachix/install-nix-action@v31
+      with:
+        extra_nix_config: |
+          experimental-features = nix-command flakes
+    - name: Prewarm flake devShell
+      run: |
+        # use the flake devShell defined in ./flake.nix to fetch deps
+        nix develop --command true
+      shell: bash
 
     - name: Install Nix
       uses: cachix/install-nix-action@v31

.github/workflows/check.yml

@@ -11,6 +11,9 @@ jobs:
   lint:
     runs-on: ubuntu-latest
     steps:
+      - name: Checkout
+        uses: actions/checkout@v5
+
       - name: Setup checkout + Nix
         uses: ./.github/actions/install-nix
 
@@ -23,6 +26,9 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
+      - name: Checkout
+        uses: actions/checkout@v5
+
       - name: Setup checkout + Nix
         uses: ./.github/actions/install-nix