Fix Semgrep pull request event (#2728)

Instead of `pull_request` event, I have moved the semgrep SAST scan to
`pull_request_target` similar to `pr-auditor` which consumes extra
secret to clone and scan using private semgrep rules.

P.S: this Semgrep Scan should
[trigger](https://stackoverflow.com/a/77069245) after merged to main

## Test plan

CI 🟢  and external contributors should be able to execute semgrep scan
<!-- Required. See
https://sourcegraph.com/docs/dev/background-information/testing_principles.
-->

.github/workflows/semgrep.yml

@@ -1,6 +1,8 @@
 name: Semgrep - SAST Scan
 
-on: [pull_request]
+on:
+  pull_request_target:
+    types: [ closed, edited, opened, synchronize, ready_for_review ]
 
 jobs:
   semgrep: