Use foldable helpers where applicable

As discussed at some earlier point in time, some of the actions/workflow
logic does not use our foldable helpers despite being able to. Switch
them over.

Signed-off-by: Daniel Müller <deso@posteo.net>

.github/actions/build-selftests/action.yml

@@ -18,9 +18,10 @@ runs:
   steps:
     - shell: bash
       run: |
-        echo "::group::Setup Env"
+        source $GITHUB_ACTION_PATH/../../../ci/vmtest/helpers.sh
+        foldable start "Setup Env"
         sudo apt-get install -y qemu-kvm zstd binutils-dev elfutils libcap-dev libelf-dev libdw-dev python3-docutils
-        echo "::endgroup::"
+        foldable end
     - shell: bash
       run: |
         export KERNEL=${{ inputs.kernel }}