fix: don't drop oidc_id when updating users

[jmattheis]

Fixes #1009

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 74.53%. Comparing base (ad137d6) to head (d09ea4e).

Files with missing lines	Patch %	Lines
api/user.go	90.90%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1010      +/-   ##
==========================================
- Coverage   74.56%   74.53%   -0.03%     
==========================================
  Files          66       66              
  Lines        3487     3483       -4     
==========================================
- Hits         2600     2596       -4     
  Misses        688      688              
  Partials      199      199              

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