[flagd] Add log output on invalid env vars

[guidobrei]

When flagd options in env vars cannot be parsed, we should log.
Currently we silently fall back to a default value.

TBD: Which log level is appropriate?

I'm in favor for ERROR. Reason: It's a misconfiguration that can lead to unexpected behavior and requires user intervention.

Originally posted by @chrfwow in #1624 (comment)