ci: Use github action's cache (#319)

We don't have many crate deps, but this is free and reduces sccache load slightly.

.github/workflows/ci.yml

@@ -60,6 +60,23 @@ jobs:
           python-version: "2.7.x"
           architecture: x64
 
+      # Cache https://github.com/actions/cache/blob/master/examples.md#rust---cargo
+      - name: Cache cargo registry
+        uses: actions/cache@v1
+        with:
+          path: ~/.cargo/registry
+          key: ${{ runner.os }}-cargo-registry-${{ hashFiles('**/Cargo.lock') }}
+      - name: Cache cargo index
+        uses: actions/cache@v1
+        with:
+          path: ~/.cargo/git
+          key: ${{ runner.os }}-cargo-index-${{ hashFiles('**/Cargo.lock') }}
+      - name: Cache cargo build
+        uses: actions/cache@v1
+        with:
+          path: target
+          key: ${{ runner.os }}-cargo-build-target-${{ hashFiles('**/Cargo.lock') }}
+
       - name: Install and start sccache
         shell: pwsh
         working-directory: ${{ runner.temp }}