Fix example YAMLs to call the medev evaluator

[jhrozek]

These allow to exercise the PR machinery without having to open PRs all the time e.g. with:

AUTH_TOKEN="blah" go run cmd/dev/main.go rule --policy examples/github/policies/pr_vuln_check.yaml --rule-type examples/github/rule-types/pr_vulnerability_check.yaml --entity examples/github/pull_request.yaml

[jhrozek]

@teodor-yanev you might want to use this to test your go ingestor patch. But feel free to just dump what you have to a git branch and move on to the other epics like tracing.