CI: Remove CI test runs from forks of matplotlib

This will limit the tests to the main repository and reduce CI waste.

Author: greglucas

.github/workflows/tests.yml

@@ -14,6 +14,7 @@ env:
 
 jobs:
   test:
+    if: "github.repository == 'matplotlib/matplotlib' && !contains(github.event.head_commit.message, '[ci skip]') && !contains(github.event.head_commit.message, '[skip ci]') && !contains(github.event.head_commit.message, '[skip github]')"
     name: "Python ${{ matrix.python-version }} on ${{ matrix.os }} ${{ matrix.name-suffix }}"
     runs-on: ${{ matrix.os }}