Skip unused dependency check to avoid depcheck open file spike

[mikrostew]

From npm-check docs:

-s, --skip-unused

By default npm-check will let you know if any of your modules are not being used by looking at require statements in your code.

This option will skip that check.

Checkup doesn't use this info when calculating out-of-date dependencies, and it causes a big spike in open files due to how depcheck works internally.

This PR is a small change to avoid that.