Change the workflow event to pull_request_target

we are changing this to be able to access the secrets needed to run
integration tests and claim environment for it

Signed-off-by: João Pereira <joaod@vmware.com>

.github/workflows/tests-unit.yml

@@ -7,7 +7,7 @@ on:
       - "v9.*"
       - "v8.*"
       - "v7.*"
-  pull_request:
+  pull_request_target:
     branches:
       - main
       - v9