Reenable old nightly cloud TPU for workflow_dispatch

The workflow file became invalid when it lost all of its "on:" conditions.  Reenable workflow_dispatch to prevent notifications for failures

PiperOrigin-RevId: 753280799

Author: MichaelHudgins

.github/workflows/cloud-tpu-ci-nightly.yml

@@ -13,10 +13,10 @@
 name: CI - Cloud TPU (nightly)
 # Disable the schedule; Slated for removal, the new test workflow is in
 # "wheel_tests_nightly_release.yml"
-# on:
+on:
 #   schedule:
 #     - cron: "0 2,14 * * *" # Run at 7am and 7pm PST
-#   workflow_dispatch: # allows triggering the workflow run manually
+  workflow_dispatch: # allows triggering the workflow run manually
 
 # This should also be set to read-only in the project settings, but it's nice to
 # document and enforce the permissions here.