Automatically run on push to main (#14)

Author: TomHennen

.github/workflows/local_attest.yml

@@ -1,6 +1,10 @@
 name: Attest to this repo's source
 on:
-  # TODO: Update to happen automatically as appropriate
+  push:
+    branches: [ "main" ]
+  # TODO: Remove workflow_dispatch so that we only create VSAs for protected branches
+  # contemporaneously to the change.  (And probably really check in
+  # create_slsa_source_vsa.yml).
   workflow_dispatch:
 
 jobs: