build: Ensure all required checks run on merge_group

And clean up some of the whitespace formatting so that the "name" and
"on" are grouped away from the "jobs".

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

.github/workflows/shlint.yaml

@@ -1,7 +1,7 @@
 name: Shell scripts check
-
 on:
   pull_request:
+  merge_group:
   push:
     branches:
       - main