GitHub Actions: Remove the deprecated set-output command (#72)

Fixes warnings at bottom right of https://github.com/pytest-dev/unittest2pytest/actions/runs/12145268432

Ref: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands

.github/workflows/test.yml

@@ -26,7 +26,7 @@ jobs:
       - name: Get pip cache dir
         id: pip-cache
         run: |
-          echo "::set-output name=dir::$(pip cache dir)"
+          echo "dir=$(pip cache dir)" >> $GITHUB_OUTPUT
 
       - name: Cache
         uses: actions/cache@v4