Make github action shellcheck run on PRs

Modify the shell check workflow to operate on the same events as the
swupd tests.

Signed-off-by: William Douglas <william.douglas@intel.com>

Author: William Douglas

.github/workflows/shellcheck-all.yml

@@ -1,8 +1,6 @@
 name: Shellcheck
 
-on:
-  push:
-    branches: [master]
+on: [push, pull_request]
 
 env:
   RUNNING_IN_CI: true