Use "on: workflow_call", not "workflow_dispatch"

Reusable/shared workflows need to trigger on workflow_call rather than
workflow_dispatch (which is used for non-shared gerrit workflows).

Signed-off-by: Eric Ball <eball@linuxfoundation.org>
Change-Id: I5d45a12c7ace04c88101e642c9d42affa81cfb84
diff --git a/.github/workflows/verify-unit-tests.yaml b/.github/workflows/verify-unit-tests.yaml
index 54c77b3..26034c3 100644
--- a/.github/workflows/verify-unit-tests.yaml
+++ b/.github/workflows/verify-unit-tests.yaml
@@ -5,7 +5,7 @@
 
 # yamllint disable-line rule:truthy
 on:
-  workflow_dispatch:
+  workflow_call:
     inputs:
       # Gerrit-to-Github defaults
       GERRIT_BRANCH: