ci(insider): use insider branch for dispatch [skip ci]

.github/workflows/insider-spearhead.yml

@@ -78,4 +78,10 @@ jobs:
         uses: peter-evans/repository-dispatch@v3
         with:
           event-type: 'insider'
+          client-payload: |-
+            {
+              "repo": {
+                "branch": "insider"
+              }
+            }
         if: github.event.inputs.dont_dispatch != 'true' && (env.SHOULD_BUILD == 'yes' || github.event.inputs.force_dispatch == 'true')