cfg: respect VOUCH_LOGLEVEL when no config file value is set

[c-tonneslan]

Fixes #540.

`Logging.configure()` reset `Cfg.LogLevel` back to the default whenever `viper.IsSet` on the config-file key was false. `envconfig` (which is what reads `VOUCH_LOGLEVEL`) writes straight into `Cfg.LogLevel` and never goes through viper, so a user who set `VOUCH_LOGLEVEL=debug` without also putting `logLevel` in the config file always ended up back on info.

Also check the matching env var before clobbering the value, so an env-only configuration is honored. The config-file path still wins when both are present, matching the documented precedence.

[bnfinet]

@c-tonneslan very much appreciate the contribution.

Regarding...

The config-file path still wins when both are present,

Please take a look at...

vouch-proxy/pkg/cfg/cfg.go

Line 175 in b683f60

// The priority of the sources is the following:

The ENV var should take precedence.

[c-tonneslan]

Good catch, sorry about that. You're right, the documented precedence is env > config and the actual order in Configure already does that: parseConfigFile populates Cfg.LogLevel and then configureFromEnv overwrites it via envconfig.Process. My fix doesn't change that flow, it just stops Logging.configure from clobbering the env value back to the default when no config-file value is set.

The line I wrote in the PR description ("config-file path still wins when both are present") was backwards. Want me to amend the commit message or push a description-only fixup?