update-nightly fails on MacOS

[scottyeck]

Per the README...

MacOS (Untested, if you found issues please report them!)

...so here's an issue.

$ nvenv update-nightly
[LOG] Downloading version nightly update ...
######################################################################################################################################################################################################################################################### 100.0%
[LOG] Updating version nightly ...
cp: the -R and -r options may not be specified together.
[ERR] Failed to update Nvim.

On OSX, cp -ar is redundant, as cp -a suffices. Changing the flags on this line has the desired effect.

It appears that this may also be the case on Linux, but I don't trust my reading of the man-page as I'm not a Linux user. If it's not, we could likely get around this with the same system-sniffing checks used here. Happy to issue a PR if you'd like!

[siduck]

works fine here