Add Rust cache to CI

.github/workflows/doc-test.yml

@@ -26,6 +26,7 @@ jobs:
           cargo update -p cosmwasm-schema-derive
           cargo update -p cosmwasm-schema
           cargo update -p cosmwasm-std
+      - uses: Swatinem/rust-cache@v2
       - name: Extract all the tests
         working-directory: ./docs-test-gen
         run: cargo run