Update downstream-artifacts.yml (#3613)

.github/workflows/downstream-artifacts.yml

@@ -1,6 +1,8 @@
 name: Build downstream artifacts
 on:
-    pull_request: {}
+    # We only want the Rust Crypto Cypress tests on merge queue to prevent regressions
+    # from creeping in. They take a long time to run and consume 4 concurrent runners.
+    # Anyone working on Rust Crypto is able to run the tests locally if required.
     merge_group:
         types: [checks_requested]