Move building the book to its own workflow since it takes forever to install mdbook

.github/workflows/build-book.yml

@@ -0,0 +1,38 @@
+name: Build book
+
+on:
+  push:
+    branches: [ master ]
+    paths:
+      'book/**'
+  pull_request:
+    branches: [ master ]
+    paths:
+      'book/**'
+
+env:
+  CARGO_TERM_COLOR: always
+
+jobs:
+  build:
+    runs-on: ubuntu-20.04
+    steps:
+    - name: Set CARGO_TARGET_DIR
+      run: echo "CARGO_TARGET_DIR=$HOME/target" >> $GITHUB_ENV
+    - name: Install build tools
+      run: sudo apt-get update && sudo apt-get install build-essential binutils-arm-none-eabi libelf-dev zip -y
+    - uses: actions/checkout@v2
+    - name: Cache
+      uses: actions/cache@v2.1.7
+      with:
+        path: |
+          ~/.cargo/registry
+          ~/.cargo/git
+          ~/target
+        key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
+
+    - name: Install mdbook
+      run: cargo install mdbook
+    - name: Build the book
+      run: mdbook build
+      working-directory: book

.github/workflows/rust.yml

@@ -31,12 +31,7 @@ jobs:
       run: cargo install --path mgba-test-runner --verbose
     - name: Install gbafix
       run: cargo install gbafix
-    - name: Install mdbook
-      run: cargo install mdbook
     - name: Build and test all crates
       run: bash .github/scripts/run-branch-build.sh
     - name: Build example projects
       run: bash .github/scripts/build-example-gba-files.sh
-    - name: Build the book
-      run: mdbook build
-      working-directory: book