Add cargo cache to speed up builds

.github/workflows/epub.yml

@@ -16,6 +16,18 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - name: Cache cargo registry
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cargo/bin/
+            ~/.cargo/registry/index/
+            ~/.cargo/registry/cache/
+            ~/.cargo/git/db/
+          key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-cargo-
+
       - name: Install mdbook
         uses: peaceiris/actions-mdbook@v2
         with: