chore: Add manual trigger for nightly_dafny.yml (#629)

* Add manual trigger for nightly_dafny.yml

* chore: Add manual trigger for nightly_dafny.yml

Author: robin-aws

.github/workflows/nighly_dafny.yml

@@ -9,6 +9,7 @@ on:
     # but this might need to be tweaked:
     # https://github.com/dafny-lang/dafny/blob/master/.github/workflows/deep-tests.yml#L16
     - cron: "30 16 * * *"
+  workflow_dispatch:
 
 jobs:
   dafny-nightly-verification: