Fix sourcing of helpers.sh in coverity workflow

The path to the helpers.sh script to source was put one level too deep
by cfbd763ef8 ("Use foldable helpers where applicable") and the
GITHUB_ACTION_PATH variable is not actually defined in a workflow.

Fix up both issues.

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

.github/workflows/coverity.yml

@@ -15,7 +15,7 @@ jobs:
       - uses: ./.github/actions/setup
       - name: Run coverity
         run: |
-          source $GITHUB_ACTION_PATH/../../../ci/vmtest/helpers.sh
+          source "${GITHUB_WORKSPACE}"/ci/vmtest/helpers.sh
           foldable start "Setup CI env"
           source /tmp/ci_setup
           export COVERITY_SCAN_NOTIFICATION_EMAIL="${AUTHOR_EMAIL}"