Add flake check and push

.github/workflows/build.yml

@@ -1,15 +1,13 @@
-name: build
+name: check and push
 on:
   push:
     branches:
       - master
   pull_request:
-    branches:
-      - master
 jobs:
-  build:
+  stack-test:
+    name: stack test
     runs-on: ubuntu-latest
-    name: Haskell GHC
     steps:
       - uses: actions/checkout@v4
       - uses: haskell-actions/setup@v2
@@ -22,6 +20,34 @@ jobs:
             ~/.stack
             .stack-work
           key: ${{ runner.os }}-${{ hashFiles('**/*.cabal','**/stack.yaml') }}
+          # If there's no match for key, fall back to these:
           restore-keys: |
             ${{ runner.os }}-
-      - run: stack build
+      - run: stack test
+
+  flake-no-push:
+    name: flake check (no push)
+    if: github.event_name != 'push'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: cachix/install-nix-action@v31
+      - uses: cachix/cachix-action@v15
+        with:
+          name: stackage-infrastructure
+      - run: nix build
+      - run: nix flake check
+
+  flake-push:
+    name: flake check and push
+    if: github.event_name == 'push'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: cachix/install-nix-action@v31
+      - uses: cachix/cachix-action@v31
+        with:
+          name: stackage-infrastructure
+          authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+      - run: nix build
+      - run: nix flake check