repo: create the keyrings dir if necessary

[tigarmo]

/etc/apt/keyrings is only created by Apt after a certain version; for ubuntu 18.04, the directory is not there by default, so check and create it if necessary.

• [ ] Have you followed the guidelines for contributing?
• [ ] Have you signed the CLA?
• [ ] Have you successfully run tox?

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[codecov[bot]]

Codecov Report

Merging #47 (519ef7d) into main (1b2ce9c) will increase coverage by 0.15%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #47      +/-   ##
==========================================
+ Coverage   82.16%   82.32%   +0.15%     
==========================================
  Files          12       12              
  Lines         673      679       +6     
  Branches      153      154       +1     
==========================================
+ Hits          553      559       +6     
  Misses         88       88              
  Partials       32       32              

Impacted Files	Coverage Δ
craft_archives/repo/apt_key_manager.py	98.00% <100.00%> (+0.12%)	:arrow_up:

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