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
commented
2 years ago 
Per the README...
...so here's an issue.
On OSX,
cp -aris redundant, ascp -asuffices. 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!