Try caching Cargo index to speed up 1.13 build

This one build always took several (10+) minutes on:

    Updating registry `https://github.com/rust-lang/crates.io-index`

.github/workflows/ci.yml

@@ -46,6 +46,13 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - uses: dtolnay/rust-toolchain@1.13.0
+      - name: Get timestamp for cache
+        id: date
+        run: echo ::set-output name=yearmo::$(date +%Y%m)
+      - uses: actions/cache@v1
+        with:
+          path: ~/.cargo/registry/index
+          key: cargo-registry-index-${{steps.date.outputs.yearmo}}
       - run: cd serde && cargo build --features rc
       - run: cd serde && cargo build --no-default-features
       - run: cd serde_test && cargo build