Set CI Tests workflow to run under pr-tests github environment (#4148)

Enable forked repositories to access DagsHub token, xref https://2i2c.org/blog/2025/github-action-secrets-forked-repositories/

Author: weiji14

.github/workflows/ci_tests.yaml

@@ -51,6 +51,8 @@ jobs:
   test:
     name: ${{ matrix.os }} - Python ${{ matrix.python-version }}
     runs-on: ${{ matrix.os }}
+    environment:
+      name: pr-tests
     permissions:
       id-token: write  # This is required for requesting OIDC token for codecov
     strategy: