Don't run the daily test on contributors' GitHub forks (#285)

This PR makes it so that the `test` workflow only happens on `python/pyperformance`, not on contributors' forks of the repo (unless they specifically request the workflow by manually triggering the workflow or by making a PR to their fork).

Author: AlexWaygood

.github/workflows/main.yml

@@ -17,6 +17,7 @@ jobs:
     permissions:
       pull-requests: write
     name: ${{ matrix.os }} - ${{ matrix.python }}
+    if: ${{ github.event_name != 'schedule' || (github.repository == 'python/pyperformance' && github.event_name == 'schedule') }}
     strategy:
       fail-fast: false
       matrix: