Deploy book from releases, not commits to main

Fixes #260

.github/workflows/book.yml

@@ -1,9 +1,8 @@
-name: Book
+name: Build book
 on:
   push:
     branches:
       - main
-  # Allows you to run this workflow manually from the Actions tab
   workflow_dispatch:
   pull_request:
     branches:
@@ -12,12 +11,6 @@ on:
       - .github/workflows/book.yml
       - book/**
 
-# Sets permissions of the GITHUB_TOKEN to allow deployment to GitHub Pages
-permissions:
-  contents: read
-  pages: write
-  id-token: write
-
 jobs:
   build:
     runs-on: ubuntu-latest
@@ -40,35 +33,3 @@ jobs:
         with:
           name: book
           path: book/book
-
-  deploy:
-    if:
-      github.repository == 'sourcefrog/cargo-mutants' && github.ref_name ==
-      'main'
-    environment:
-      name: github-pages
-      url: ${{ steps.deployment.outputs.page_url }}
-    concurrency: # Allow one concurrent deployment
-      group: "pages"
-      cancel-in-progress: true
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v3
-        with:
-          fetch-depth: 0
-      - name: Install mdbook
-        uses: taiki-e/install-action@v2
-        with:
-          tool: mdbook, mdbook-linkcheck
-      - name: Setup Pages
-        uses: actions/configure-pages@v2
-      - name: Build book
-        run: |
-          mdbook build book
-      - name: Upload artifact
-        uses: actions/upload-pages-artifact@v1
-        with:
-          path: "book/book/html"
-      - name: Deploy to GitHub Pages
-        id: deployment
-        uses: actions/deploy-pages@v1

.github/workflows/release-book.yml

@@ -0,0 +1,44 @@
+# Publish the book to https://mutants.rs/ when a new release is created.
+name: Release book
+on:
+  release:
+    types:
+      - published
+  workflow_dispatch:
+
+# Sets permissions of the GITHUB_TOKEN to allow deployment to GitHub Pages
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+jobs:
+  deploy:
+    if: github.repository == 'sourcefrog/cargo-mutants'
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    concurrency: # Allow one concurrent deployment
+      group: "pages"
+      cancel-in-progress: true
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          fetch-depth: 0
+      - name: Install mdbook
+        uses: taiki-e/install-action@v2
+        with:
+          tool: mdbook, mdbook-linkcheck
+      - name: Setup Pages
+        uses: actions/configure-pages@v2
+      - name: Build book
+        run: |
+          mdbook build book
+      - name: Upload artifact
+        uses: actions/upload-pages-artifact@v1
+        with:
+          path: "book/book/html"
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v1