Improve cargo cache with static key and skip reinstall if exists

.github/workflows/epub.yml

@@ -16,7 +16,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: Cache cargo registry
+      - name: Cache cargo
         uses: actions/cache@v4
         with:
           path: |
@@ -24,9 +24,9 @@ jobs:
             ~/.cargo/registry/index/
             ~/.cargo/registry/cache/
             ~/.cargo/git/db/
-          key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
+          key: ${{ runner.os }}-cargo-mdbook-pandoc-v1
           restore-keys: |
-            ${{ runner.os }}-cargo-
+            ${{ runner.os }}-cargo-mdbook-pandoc-
 
       - name: Install mdbook
         uses: peaceiris/actions-mdbook@v2
@@ -49,7 +49,11 @@ jobs:
 
       - name: Install mdbook-pandoc
         run: |
-          cargo install mdbook-pandoc
+          if ! command -v mdbook-pandoc &> /dev/null; then
+            cargo install mdbook-pandoc
+          else
+            echo "mdbook-pandoc already installed"
+          fi
 
       - name: Build Book
         run: mdbook build