ci: restore incremental oj-verify by fetching full history

actions/checkout@v4 defaults to a shallow clone (fetch-depth: 1), which
broke oj-verify's timestamp-based change detection (regressed in #234, when
checkout went v1 -> v4). Pin fetch-depth: 0 so only changed tests re-run.

Author: cameroncuster

.github/workflows/programming_team_code_ci.yml

@@ -10,6 +10,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v4
+      with:
+        fetch-depth: 0
     - name: Set up Python
       uses: actions/setup-python@v5
     - name: Make preinstalled g++-14 the default g++