[ci] fix notify/label actions for forks (#32725)

Need this to run against target for forks to get the notification.

This job does not checkout the code in the PR, so it's safe to run from
the target.

Also fixes failing checks on PRs:

<img width="870" alt="Screenshot 2025-03-24 at 3 28 30 PM"
src="https://github.com/user-attachments/assets/add78287-6449-4e48-9376-f3b360d2607c"
/>

.github/workflows/compiler_discord_notify.yml

@@ -1,7 +1,7 @@
 name: (Compiler) Discord Notify
 
 on:
-  pull_request:
+  pull_request_target:
     types: [opened, ready_for_review]
     paths:
       - compiler/**

.github/workflows/runtime_discord_notify.yml

@@ -1,7 +1,7 @@
 name: (Runtime) Discord Notify
 
 on:
-  pull_request:
+  pull_request_target:
     types: [opened, ready_for_review]
     paths-ignore:
       - compiler/**

.github/workflows/shared_label_core_team_prs.yml

@@ -1,7 +1,7 @@
 name: (Shared) Label Core Team PRs
 
 on:
-  pull_request:
+  pull_request_target:
 
 permissions: {}