ci: run benchmarks on push to main

Enable continuous performance tracking by triggering benchmarks on main
branch pushes in addition to release tags.

Author: mlwelles

.github/workflows/ci-pydgraph-benchmarks.yml

@@ -1,6 +1,8 @@
 name: ci-pydgraph-benchmarks
 on:
   push:
+    branches:
+      - main
     tags:
       - v[0-9]+.[0-9]+.[0-9]+*