ci: update workflow trigger to pull_request

Use pull_request instead of pull_request_target so checks
run against PR code rather than the base branch.

Signed-off-by: Srikanth Muppandam <smuppand@qti.qualcomm.com>

Author: smuppand

.github/workflows/qcom-preflight-checks.yml

@@ -1,6 +1,6 @@
 name: Qualcomm Preflight Checks
 on:
-  pull_request_target:
+  pull_request:
     branches: [ "main" ]
   push:
     branches: [ "main" ]