fix(ncu-config): do not override encrypted values silently

[aduh95]

It's deceivingly easy to end up with a clear-text value when trying to modify an existing encrypted config entry. I think throwing in this case is a safer behavior.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.58%. Comparing base (7342aff) to head (42dc752).

Files with missing lines	Patch %	Lines
lib/config.js	50.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1006      +/-   ##
==========================================
- Coverage   80.65%   80.58%   -0.08%     
==========================================
  Files          39       39              
  Lines        4746     4754       +8     
==========================================
+ Hits         3828     3831       +3     
- Misses        918      923       +5     

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