Deal with trailing slashes in the `url` property of `repositories.yaml` entries

[Dexterp37]

We have been bitten by trailing slashes in the url property of repositories.yaml entries in the past, always resulting in broken probe-scraper runs, with #458 being its last instance.

We should probably add some resilience to this logic within the probe-scraper, to either check and forbid trailing slashes in the url property or gracefully handle them at runtime.