ci: fix number of runs per branch push

.gitea/workflows/check.yml

@@ -3,7 +3,11 @@
 # https://github.com/prefix-dev/setup-pixi@v0.9.4.
 # The tools need Python 3.14, which pixi.toml pins, so one environment covers every supported Python.
 name: check
-on: [push, pull_request]
+on:
+  push:
+    branches: [main]
+  pull_request:
+
 jobs:
   check:
     runs-on: linux-base