add format unknown

[DmitriyMusatkin]

Issue #, if available:

Description of changes:
SecItemImport overrides import format after the call, so we need to reset it to unknown before every call
https://developer.apple.com/documentation/security/1395728-secitemimport?language=objc

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (029587f) 79.72% compared to head (384acf7) 79.72%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #606   +/-   ##
=======================================
  Coverage   79.72%   79.72%           
=======================================
  Files          27       27           
  Lines        5893     5893           
=======================================
  Hits         4698     4698           
  Misses       1195     1195           

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