ci: cache Enzyme and use rustbook@main

.github/workflows/enzyme-ci.yml

@@ -35,6 +35,15 @@ jobs:
         with:
           path: build/build/x86_64-unknown-linux-gnu/llvm
           key: ${{ matrix.os }}-llvm-${{ steps.llvm-commit.outputs.HEAD }}
+      - name: Get Enzyme commit hash
+        id: enzyme-commit
+        run: echo "HEAD=$(git -C src/tools/enzyme rev-parse HEAD)" >> $GITHUB_OUTPUT
+      - name: Cache Enzyme
+        id: cache-enzyme
+        uses: actions/cache@v4
+        with:
+          path: build/build/x86_64-unknown-linux-gnu/enzyme
+          key: ${{ matrix.os }}-enzyme-${{ steps.enzyme-commit.outputs.HEAD }}
       - name: Build
         run: |
           mkdir -p build
@@ -47,7 +56,7 @@ jobs:
         uses: actions/checkout@v4
         with:
           repository: EnzymeAD/rustbook
-          ref: mdbook-test
+          ref: main
           path: rustbook
       - name: test Enzyme/rustbook
         working-directory: rustbook