examples: Only cache non-redundant cargo state

As the packed crate sources are stored under ~/.cargo/registry/cache,
it's redundant to archive the unpacked sources.

The sufficient local state of the git dependencies is in
~/.cargo/git/db. Cargo automatically checks out the source
trees if missing.

examples.md

@@ -432,8 +432,9 @@ When dependencies are installed later in the workflow, we must specify the same
   uses: actions/cache@v2
   with:
     path: |
-      ~/.cargo/registry
-      ~/.cargo/git
+      ~/.cargo/registry/index
+      ~/.cargo/registry/cache
+      ~/.cargo/git/db
     key: cargo-deps-${{ hashFiles('**/Cargo.lock') }}
 - name: Cache cargo build
   uses: actions/cache@v2