This is a follow-up to 8d7f3c9dbd and
ae48735c53.
Running the config validation in the build sandbox is impossible and
will fail when using `cfg.configFile` or `-config.expand-env=true`.
`cfg.configFile` is a string of a path which is simply not available to
the build sandbox.
Similarly, one may opt to use `cfg.configuration` with environment
variables in combination with `-config.expand-env=true`.
The environment variables referenced that way are also not available
in the build sandbox.
So we skip the validation when it's impossible (`cfg.configFile`) or
likely impossible (`-config.expand-env=true`).
An alternative approach would be something like nixos/prometheus'
`services.prometheus.checkConfig` that takes a boolean and makes
toggling the config validation user-facing.
100c1501e1