Exclusive installation mode

[dkartsyn]

When fnm used in CI/CD parallel pipeline we can get error:

$ fnm install v22.20.0
Installing Node v22.20.0 (x64)
error: Can't download the requested binary: Directory not empty (os error 39)

Because other process installing node right now.

Please add exclusive installation flag (lock file + check dir and pause) or some another mechanism.

Thank you.