FIX: update how fetch-kaiju-db action discovers DB URLs

[misialq]

Closes #197.

To test, execute the action with a DB of choice (viruses, plasmids or fungi are some good, small DBs to try):

qiime moshpit fetch-kaiju-db \
  --p-database-type<db type> \
  --o-database db.qza \
  --verbose

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.00%. Comparing base (1bb3e46) to head (55716d8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #201      +/-   ##
==========================================
- Coverage   98.56%   98.00%   -0.56%     
==========================================
  Files          85      155      +70     
  Lines        4882     9684    +4802     
  Branches        0      423     +423     
==========================================
+ Hits         4812     9491    +4679     
- Misses         70      140      +70     
- Partials        0       53      +53     

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[ChristosMatzoros]

Hey @misialq,

I tested it with the options (viruses, plasmids, fungi), and everything's working great. The code looks good to me. Go ahead and merge when you're ready!