ci: Do not run Sonarcloud on pull requests

The required tokens used to authenticate with Sonarcloud are not
accessible from within pull requests that originate in other forks.

.github/workflows/sonarcloud.yml

@@ -2,14 +2,12 @@
 name: Sonarcloud
 
 on:
+  # Do not run on pull-request because the required tokens
+  # cannot be read from there.
   push:
     paths-ignore:
       - "wiki/**"
       - "**.md"
-  pull_request:
-    paths-ignore:
-      - "wiki/**"
-      - "**.md"
 
 env:
   dotnetVersion: "6.0.x"