fix: add an output containing a pass/fail result

[erzz]

Closes #412

This adds a little basic logic to provide an output that gives the real result of the scan (findings or not) in a simple pass/fail format.

It still honours the exit code requested via the exit-code input in terms of pass/fail of the trivy command whilst providing the ability to take conditional actions later in a workflow (upload sarif, prepare reports in other formats using convert, uploading to another system etc)

Example from a workflow in a private repo using this branch:

Debug step in my workflow

- name: Check Output
        env:
          RESULT: ${{ steps.scan.outputs.result }}
        run: |
          echo "Trivy scan result output: $RESULT"