chore: Fix the workflow that adds the interface-spec label (#5243)

When a PR is submitted from a fork from an external contributor, the workflow that attempts to add the `interface-spec` label fails, e.g. on this [PR](#5240).

According to actions/labeler#36 (comment) this should be fixed by changing the type of action that triggers the workflow from `pull_request` to `pull_request_target` as this adds the necessary write permissions through a github token.

.github/workflows/interface-spec.yml

@@ -1,6 +1,6 @@
 name: Interface Specification
 on:
-  pull_request:
+  pull_request_target:
     paths:
       - .github/workflows/interface-spec.yml
       - docs/references/http-gateway-protocol-spec.md