build: add cache to roller CI (#9)

.github/workflows/roller.yml

@@ -7,12 +7,14 @@ on:
       - staging
     paths:
       - 'roller/**'
+      - '.github/workflows/roller.yml'
   pull_request:
     branches:
       - main
       - staging
     paths:
       - 'roller/**'
+      - '.github/workflows/roller.yml'
 
 defaults:
   run:
@@ -33,6 +35,21 @@ jobs:
         go-version: 1.17.x
     - name: Checkout code
       uses: actions/checkout@v2
+    - name: Cache cargo registry
+      uses: actions/cache@v2
+      with:
+        path: ~/.cargo/registry
+        key: ${{ runner.os }}-roller-cargo-registry-${{ hashFiles('roller/roller/prover/rust/Cargo.lock') }}
+    - name: Cache cargo index
+      uses: actions/cache@v2
+      with:
+        path: ~/.cargo/git
+        key: ${{ runner.os }}-roller-cargo-index-${{ hashFiles('roller/roller/prover/rust/Cargo.lock') }}
+    - name: Cache cargo target
+      uses: actions/cache@v2
+      with:
+        path: /home/runner/work/scroll/scroll/roller/roller/prover/rust/target
+        key: ${{ runner.os }}-roller-cargo-build-target-${{ hashFiles('roller/roller/prover/rust/Cargo.lock') }}
     - name: Test
       run: |
         make roller