ci: Use GITHUB_OUTPUT envvar instead of set-output command (#331)

`save-state` and `set-output` commands used in GitHub Actions are deprecated and [GitHub recommends using environment files](https://github.blog/changelog/2023-07-24-github-actions-update-on-save-state-and-set-output-commands/).

This PR updates the usage of `set-output` to `$GITHUB_OUTPUT`

Instructions for envvar usage from GitHub docs:

https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#setting-an-output-parameter

.github/workflows/build-and-test.yml

@@ -54,7 +54,7 @@ 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: pip cache
         uses: actions/cache@v2
         with: