github: Split pages deploy & build workflow

.github/workflows/build-main.yml

@@ -0,0 +1,26 @@
+name: Build (main)
+
+on:
+  push:
+    branches: [main]
+
+# Sets permissions of the GITHUB_TOKEN to allow deployment to GitHub Pages
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+jobs:
+  build:
+    uses: ./.github/workflows/build.yml
+
+  deploy-pages:
+    needs: build
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    runs-on: ubuntu-latest
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v1

.github/workflows/build.yml

@@ -2,15 +2,10 @@ name: Build
 
 on:
   push:
+    branches: ['*', '!main']
   workflow_dispatch:
   workflow_call:
 
-# Sets permissions of the GITHUB_TOKEN to allow deployment to GitHub Pages
-permissions:
-  contents: read
-  pages: write
-  id-token: write
-
 jobs:
   build:
     strategy:
@@ -31,6 +26,7 @@ jobs:
         if: runner.os == 'macOS'
 
       # Upload build artifacts to site
+      # Deploy happens in the `deploy-site` workflow, which only runs on main
       - run: |
           mkdir -p .build/site/ci
           cp .build/coverage.svg .build/coverage.html .build/site/ci
@@ -55,14 +51,3 @@ jobs:
   #       run: |
   #         $Env:Path += ";" + (swift build --show-bin-path)
   #         echo "test" | age --encrypt --recipient age1se1qgg72x2qfk9wg3wh0qg9u0v7l5dkq4jx69fv80p6wdus3ftg6flwg5dz2dp -o secret.txt.age
-
-  deploy-pages:
-    environment:
-      name: github-pages
-      url: ${{ steps.deployment.outputs.page_url }}
-    runs-on: ubuntu-latest
-    needs: build
-    steps:
-      - name: Deploy to GitHub Pages
-        id: deployment
-        uses: actions/deploy-pages@v1