unpack on a different device than PUB_CACHE

[jonasfj]

I guess it's not surprising, I'm not sure I have a fantastic option, perhaps the best choice here is to fallback to copy, when move doesn't work.

Usually, we only move within PUB_CACHE where folders are all on the same device (one would hope 🤣)

$ mkdir /tmp/foo
$ cd /tmp/foo
$ dart pub unpack retry
Downloading retry 3.1.2 to `./retry-3.1.2`... 
Rename failed, path = '/jonasfj/.pub-cache/_temp/dirVVNBRE' (OS Error: Invalid cross-device link, errno = 18)

[sigurdm]

We could stop doing atomic extraction for unpack.