ci: Skip diff-changes if fork (#2645)

Author: daquinteroflex

.github/workflows/tidy3d-python-client-tests.yml

@@ -213,7 +213,8 @@ jobs:
       if: >- 
         matrix.python-version == '3.13' &&
         github.event_name == 'pull_request' &&
-        !contains(github.event.pull_request.labels.*.name, 'ignore_diff_coverage')
+        !contains(github.event.pull_request.labels.*.name, 'ignore_diff_coverage') &&
+        github.event.pull_request.head.repo.fork == false
       with:
         result-encoding: string
         script: |