Update and modernize GitHub actions

.github/workflows/build.yml

@@ -1,30 +1,43 @@
-name: 'build'
+name: build
+
 on:
   pull_request:
   push:
+
 jobs:
   build:
     runs-on: ubuntu-latest
+    timeout-minutes: 30
+
     steps:
-      - name: 'Checkout codebase'
-        if: github.event_name == 'push'
-        uses: actions/checkout@v2.4.0
+      - name: Checkout (push/merge)
+        if: ${{ github.event_name != 'pull_request' }}
+        uses: actions/checkout@v5
         with:
           fetch-depth: 0
 
-      - name: 'Checkout codebase'
-        if: github.event_name == 'pull_request'
-        uses: actions/checkout@v2.4.0
+      - name: Checkout (PR head)
+        if: ${{ github.event_name == 'pull_request' }}
+        uses: actions/checkout@v5
         with:
           fetch-depth: 0
           ref: ${{ github.event.pull_request.head.sha }}
 
-      - uses: cachix/install-nix-action@v14
-      - run: NIXPKGS_ALLOW_UNFREE=1 nix --experimental-features 'nix-command flakes' build -L --impure
-      - name: 'Copy book'
-        run: cp result/share/book.pdf .
-      - name: 'Upload artifact to GitHub'
-        uses: actions/upload-artifact@v2.2.4
+      - name: Install Nix
+        uses: cachix/install-nix-action@v31
+
+      - name: Build (Nix flake)
+        run: nix build -L --impure
+        env:
+          NIXPKGS_ALLOW_UNFREE: 1
+
+      - name: Copy book
+        run: cp result/share/book.pdf ./book.pdf
+
+      - name: Upload artifact
+        uses: actions/upload-artifact@v4
         with:
           name: book.pdf
           path: book.pdf
+          if-no-files-found: error
+          retention-days: 90