interpret paths from blast db arguments

nf-core/nanoclust pull request

Many thanks for contributing to nf-core/nanoclust!

Please fill in the appropriate checklist below (delete whatever is not relevant). These are the most common things requested on pull requests (PRs).

PR checklist

[x] This comment contains a description of changes (with reason)
[ ] If you've fixed a bug or added code that should be tested, add tests!
[ ] If necessary, also make a PR on the nf-core/nanoclust branch on the nf-core/test-datasets repo
[ ] Ensure the test suite passes (nextflow run . -profile test,docker).
[ ] Make sure your code lints (nf-core lint .).
[ ] Documentation in docs is updated
[ ] CHANGELOG.md is updated
[ ] README.md is updated

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