[ci]: Quick codspeed.yml tweaks to enable comparisons with master (#30752)

* Only run codspeed logic when `libs/core` is changed (for now, we'll
want to add other benchmarks later
* Also run on `master` so that we can get a reference :)

.github/workflows/codspeed.yml

@@ -1,7 +1,12 @@
 name: CodSpeed
 
 on:
+  push:
+    branches:
+        - master
   pull_request:
+    paths:
+      - 'libs/core/**'
   # `workflow_dispatch` allows CodSpeed to trigger backtest
   # performance analysis in order to generate initial data.
   workflow_dispatch: