Disable check-spelling sarif for PowerDNS/pdns

Disable check-spelling sarif for PowerDNS/pdns

- At present, it's too complicated to rely on rulesets in combination
  with `pull_request` so it makes sense to turn it off for the main
  repository.

- Leave SARIF reporting enabled by default for repositories other than
  PowerDNS/pdns.

- When active, public repositories will need to add a code scanning
  ruleset if they want to use pull requests that are not cross-forks
  and they should not accept pull requests from forks as processing
  won't work.

- For private repositories, unless you're using GHEC and paying for
  Advanced Security, you'll want to set a repository actions variable
  `DO_NOT_USE_SARIF_REPORTING` (see `/settings/variables/actions`) to
  `1` to disable SARIF.
  - This commit fixes the logic for that.