Update how output variables are stored in Actions (#206)

Replace the deprecated `set-output` in GitHub Actions for concatenating
name and values into the `GITHUB_OUTPUT` env variable.
Reference:
https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

.github/workflows/test.yml

@@ -115,7 +115,7 @@ jobs:
       - name: Get the pip cache folder
         id: pip-cache
         run: |
-          echo "::set-output name=dir::$(pip cache dir)"
+          echo "dir=$(pip cache dir)" >> $GITHUB_OUTPUT
 
       - name: Setup caching for pip packages
         uses: actions/cache@v4