ci: fetch git submodules in buf-lint workflow

third_party/api is a submodule (temporalio/api); without it the temporal
proto's external imports don't resolve and buf lint fails.

Author: rustatian

.github/workflows/buf-lint.yml

@@ -13,6 +13,8 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v4
+        with:
+          submodules: recursive
 
       - name: Set up buf
         uses: bufbuild/buf-setup-action@v1