Spurious error log when setting feature flags at startup

__Steps to reproduce:__
Described in [this comment](https://github.com/influxdata/influxdb/issues/17666#issuecomment-791956157).

__Expected behavior:__
1. Feature flags can be set via CLI, env, or config
2. When a feature flag is successfully set, no error log is displayed

__Actual behavior:__
Sometimes, attempts to override a feature flag are ignored.
Other times, the override succeeds, but a log message says it failed.

Start with the reproduction details in the linked comment, then trace how feature flags are read from the CLI, environment, and configuration during startup. Verify both successful overrides and the absence of spurious error logs across those inputs.

Written by the indexing model from the issue text.