Do not delete cached downloads when version changes

[aldenquimby]

Copied from sgraham785#17

Going to publish a fork b/c that repo appears dead

Problem context

This is an enhancement on sgraham785#16

Steps

• project A uses flyway 7.x and project B uses flyway 8.x
• npm ci project A, then npm ci project B, then npm ci project A again

Expected

• ideally the 2nd install for project A doesn't re-download the 7.x binary, because it's already been cached

Actual

• 2nd install re-downloads 7.x because the entire cache download folder is deleted whenever version changes

How does this address the problem?

• don't delete the entire download folder if there's no cached binary
• semi-related problem I hit testing this locally - if lib exists from a previously failed extraction, we don't want to return after rimrafing, we need to run the extraction

Verification

• npm run test passes