Set explicit output variable on shared action.

Local tests did not need this...but apparently the runner in the cloud does.

Author: BigBlueHat

.github/actions/npm-audit-list/action.yaml

@@ -2,6 +2,10 @@ name: Output npm audit and list results as Markdown
 description: >
   A reusable action to output `npm audit` and a filtered `npm list` (based on
   the audit results) in Markdown.
+outputs:
+  result:
+    description: Combined npm audit/list reports
+    value: ${{ steps.combined_report.outputs.result }}
 runs:
   using: composite
   steps: