github: check-runs message size limit not respected

We use the check-runs GitHub API to post check results. However, when the result message to be posted is large, GitHub rejects it with an error like this:

hosting service error: host error: github error: {"message":"Invalid request.\n\nOnly 65535 characters are allowed; 2668662 were supplied.","documentation_url":"https://docs.github.com/rest/reference/checks#create-a-check-run"}