Add cache reuse assertion to pnpm 11 test

Assert that pnpm reports reused packages on rebuild to verify
the store-dir cache is actually being used.

Author: colincasey

spec/ci/pnpm_spec.rb

@@ -127,6 +127,7 @@
 
       expect(app.output).to include("Restoring cache")
       expect(app.output).to include("Running 'pnpm install' with pnpm-lock.yaml")
+      expect(app.output).to match(/reused \d*[1-9]\d*/)
       expect(app.output).to include("Pruning devDependencies")
       expect(successful_body(app).strip).to eq("Hello from pnpm 11")
     end