CI: Integrate workflow_triggers for platform-samples from cartland's fork (#297)

* Update workflow triggers
- build.yml: Add workflow_dispatch, specify branches for pull_request.
- issues-stale.yml: Add workflow_dispatch trigger.

* Remove branch filter for pull_request trigger

Author: cartland

.github/workflows/build.yml

@@ -1,6 +1,7 @@
 name: Build
 
 on:
+  workflow_dispatch:
   push:
     branches:
       - main

.github/workflows/issues-stale.yml

@@ -1,5 +1,6 @@
 name: 'Close stale issues and PRs'
 on:
+  workflow_dispatch:
   schedule:
     - cron: '15 3 * * *'