add(action): create composite action to install Nix with flakes enabled

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

@@ -0,0 +1,19 @@
+name: install-nix
+description: Composite action to checkout the repo and install Nix with flakes enabled
+runs:
+  using: composite
+  steps:
+    - name: Checkout
+      uses: actions/checkout@v5
+
+    - name: Install Nix
+      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