keys: handle keyring path parents not existing

[lengau]

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This is needed for https://github.com/canonical/rockcraft/pull/217

[codecov[bot]]

Codecov Report

Merging #50 (d34b469) into main (7983916) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #50      +/-   ##
==========================================
+ Coverage   88.05%   88.07%   +0.01%     
==========================================
  Files          13       13              
  Lines         628      629       +1     
  Branches      124      125       +1     
==========================================
+ Hits          553      554       +1     
  Misses         61       61              
  Partials       14       14              

Impacted Files	Coverage Δ
craft_archives/repo/apt_key_manager.py	98.21% <100.00%> (+0.01%)	:arrow_up: