Use BuildGrid's custom YAML parser for the CAS janitor

This replaces our use of the standard yaml parser with the custom parser used for parsing BuildGrid's server config. This allows us to use some useful YAML tags like !expand-vars to avoid directly writing secrets in the config file.

It would perhaps be nice to not use the full parser, but I'd prefer to leave making this reuse cleaner to a future MR which tackles improving our config parsing approach in general.