update build workflow to run on cargo updates

Author: jonian

.github/workflows/build.yml

@@ -8,6 +8,8 @@ on:
       - 'ext/**'
       - 'lib/**'
       - 'test/**'
+      - 'Cargo.toml'
+      - 'Cargo.lock'
       - '*.gemspec'
   pull_request:
     branches: [master]