github/workflows: disable push workflow on pr branches

.github/workflows/user_project_ci.yml

diff --git a/.github/workflows/user_project_ci.yml b/.github/workflows/user_project_ci.yml
index a39a0db..63e864f 100644
--- a/.github/workflows/user_project_ci.yml
+++ b/.github/workflows/user_project_ci.yml
@@ -2,6 +2,8 @@
 
 on:
   push:
+    branches:
+      - main
   pull_request:
   workflow_dispatch: