Fix possible exceptions by matching the type of Snapshot job to the spec

[smorimoto]

https://docs.github.com/en/rest/dependency-graph/dependency-submission#create-a-snapshot-of-dependencies-for-a-repository

Error: HTTP Status 422 for request POST https://api.github.com/repos/smorimoto/opam-dependency-submission/dependency-graph/snapshots
Error: Response body:
{
  "message": "Invalid request.\n\nInvalid property /job/id: `3116879171` is not of type `string`.",
  "documentation_url": "https://docs.github.com/rest/reference/dependency-graph#create-a-snapshot-of-dependencies-for-a-repository"
}
Error: Invalid request.

[febuiles]

Thank you, this looks good!