Merge pull request jupyterhub#837 from consideRatio/pr/set-output

ci: fix deprecation of set-output in github workflows

.github/workflows/integration-test.yaml

@@ -50,12 +50,12 @@ jobs:
           matrix_post_filter=$(
             echo "$matrix_include_pre_filter" \
               | yq e --output-format=json '.' - \
-              | jq '{"include": map( . | select(.dont_run_on_ref != "${{ github.ref }}" ))}'
+              | jq -c '{"include": map( . | select(.dont_run_on_ref != "${{ github.ref }}" ))}'
           )
-          echo ::set-output name=matrix::$(echo "$matrix_post_filter")
+          echo "matrix=$matrix_post_filter" >> $GITHUB_OUTPUT
 
           echo "The subsequent job's matrix are:"
-          echo $matrix_post_filter | jq '.'
+          echo $matrix_post_filter | jq -C '.'
         env:
           matrix_include_pre_filter: |
             - name: "Int. tests: Ubuntu 18.04, Py 3.6"