Fix log level selection

[sosthene-nitrokey]

In #176 I wanted to allow RUST_LOG overriding the config file, but it also made the config file be always ignored when file logging was enabled

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (92f196c) 90.37% compared to head (56358b2) 90.40%.

Files	Patch %	Lines
pkcs11/src/config/logging.rs	90.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #184      +/-   ##
==========================================
+ Coverage   90.37%   90.40%   +0.03%     
==========================================
  Files          31       31              
  Lines        6627     6638      +11     
==========================================
+ Hits         5989     6001      +12     
+ Misses        638      637       -1     

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