Convert object payloads to string

[KevinCarterDev]

The GitHub documentation for Deployments says that the payload can be "object or string". When setting the payload to an object this Action fails with

Error: Unable to process file command 'output' successfully.
Error: Invalid format '  "namespace": "default"'

This is due to the output being placed across multiple lines

+ jq -r .deployment.payload /github/workflow/event.json
+ echo 'payload={
  "namespace": "default"
}'

By adding the tostring function the error is fixed.

+ jq -r '.deployment.payload|tostring' /github/workflow/event.json
+ echo 'payload={"namespace":"default"}'

[billputer]

@rsotnychenko Any chance of merging this PR? It's necessary for our workflow.