GH Actions: Update set-output to use the new environment files

The old approach is deprecated.

.github/workflows/nightlies.yml

@@ -50,7 +50,7 @@ jobs:
         run: |
           set -euox pipefail
           ls -aul ~ ~/artifacts
-          echo "::set-output name=artifacts_directory::$HOME/artifacts"
+          echo "artifacts_directory=$HOME/artifacts" >> $GITHUB_OUTPUT
 
       #################################################################
       # Create the new release using the downloaded artifacts

.github/workflows/runnable_cxx.yml

@@ -152,12 +152,12 @@ jobs:
       run: |
         # For pull requests, base_ref will not be empty
         if [ ! -z ${{ github.base_ref }} ]; then
-            echo "::set-output name=branch::${{ github.base_ref }}"
+            echo "branch=${{ github.base_ref }}" >> $GITHUB_OUTPUT
         # Otherwise, use whatever ref we have:
         # For branches this in the format 'refs/heads/<branch_name>',
         # and for tags it is refs/tags/<tag_name>.
         else
-            echo "::set-output name=branch::${{ github.ref }}"
+            echo "branch=${{ github.ref }}" >> $GITHUB_OUTPUT
         fi
 
     #########################################