Specify used secret in release app workflow

This was not needed before because all the apps were in the same GitHub
org. Now we are also having apps in the NordicSemi org, so this seems to
be necessary.

Author: datenreisender

.github/workflows/release-app.yml

@@ -12,6 +12,9 @@ on:
                 type: number
             doc_bundle_name:
                 type: string
+        secrets:
+            COM_NORDICSEMI_FILES_PASSWORD_SWTOOLS_FRONTEND:
+                required: false
 
 env:
     ARTIFACT_NAME: ${{ github.event.repository.name }}