Only filter useable keys if trustmodel is not alwaystrust

[dominikschulz]

Fixes #663

[codecov]

Codecov Report

Merging #664 into master will decrease coverage by 0.07%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #664      +/-   ##
==========================================
- Coverage   64.95%   64.87%   -0.07%     
==========================================
  Files         144      144              
  Lines        8093     8102       +9     
==========================================
  Hits         5256     5256              
- Misses       2198     2205       +7     
- Partials      639      641       +2

Impacted Files	Coverage Δ
store/sub/gpg.go	40.98% <0%> (-0.68%)	⬇️
backend/crypto/gpg/cli/keyring.go	31.03% <0%> (-4.97%)	⬇️

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