examples: Reuse cargo registry cache across OSes

The registry filesystem format and the git repositories work
across platforms, no need to maintain runner-specific
caches for these.

examples.md

@@ -428,13 +428,18 @@ When dependencies are installed later in the workflow, we must specify the same
 ## Rust - Cargo
 
 ```yaml
-- uses: actions/cache@v2
+- name: Cache cargo dependencies
+  uses: actions/cache@v2
   with:
     path: |
       ~/.cargo/registry
       ~/.cargo/git
-      target
+    key: cargo-deps-${{ hashFiles('**/Cargo.lock') }}
-    key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
+- name: Cache cargo build
+  uses: actions/cache@v2
+  with:
+    path: target
+    key: ${{ runner.os }}-cargo-build-${{ hashFiles('**/Cargo.lock') }}
 ```
 
 ## Scala - SBT