Skip image scan if the image is not pushed to ECR (#159)

*Issue #, if available:*
Otherwise the workflow fails. example:
https://github.com/aws-observability/aws-otel-python-instrumentation/actions/runs/8692589074/job/23837547147

*Description of changes:*


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.github/actions/artifacts_build/action.yml

@@ -94,7 +94,8 @@ runs:
         load: ${{ inputs.load_image }}
 
     - name: Perform image scan
+      if: ${{ inputs.push_image == true || inputs.push_image == 'true' }}
       uses: ./.github/actions/image_scan
       with:
         image-ref: ${{ inputs.image_uri_with_tag }}
-        severity: 'CRITICAL,HIGH,MEDIUM,LOW,UNKNOWN'
+        severity: 'CRITICAL,HIGH,MEDIUM,LOW,UNKNOWN'