disable diff validate for non docs-builder repos (#1699)

Author: Mpdreamz

.github/workflows/preview-build.yml

@@ -245,6 +245,7 @@ jobs:
       - name: 'Validate redirect rules'
         if: >
           env.MATCH == 'true'
+          && github.repository == 'elastic/docs-builder'
           && (
             steps.deployment.outputs.result
             || (