Fix build issue.Fix node_mirror, npm_mirror not work issue.Add cleanup work when download is interrupted by user.

[Excited-ccccly]

Let me talk about this pull request from the beginning.

I live in China, therefore downloading directly from nodejs.org or GitHub is so slow which is frustrating.

I want to use a mirror to speed up downloading, but I found that neither nvm node_mirror nor nvm npm_mirror would work.Then I use ctrl+c to interrupt the download process and try again.But I found that nvm told me that the version is already installed.Then I digged into the nvm directory and found there was a folder of the version which I interrupt.

So I fix node_mirror, npm_mirror not work issue and add cleanup work when download is interrupted by user.

[coreybutler]

I really like the fact you added rollback support for interrupted downloads. Overall, this looks good. Let me run a few tests to confirm, then I'll merge and release. Thank you!

[phtwo]

There seems to be a problem.
nvm node_mirror should print the value but reset node_mirror to empty.