CI automations improvement

Switch to pull_request_target in preview-comment.yml.

Compare the updated files to merge-base trunk instead of trunk in
validate_pr.yml.

.github/workflows/preview-comment.yml

@@ -2,7 +2,7 @@
 name: Pull Request Comments
 
 on:
-    pull_request:
+    pull_request_target:
 
 # Disable permissions for all available scopes by default.
 # Any needed permissions should be configured at the job level.

validate_pr.py

@@ -85,8 +85,9 @@ def find_urls(obj):
 
 
 def get_touched_directories():
-    # Run git diff command to get the list of directories touched in the current branch
-    diff_output = os.popen('git diff --name-only origin/trunk').read()
+    # Run git diff command to get the list of directories touched in the current branch as compared to 
+    # the point where it was forked from the trunk branch
+    diff_output = os.popen('git diff --name-only $(git merge-base trunk HEAD)').read()
     touched_dirs = set()
 
     # Extract the directory paths from the diff output