nightly.sh: also push "stable" assets to vX.Y.Z tag
nightly.sh: also push "stable" assets to vX.Y.Z tag

Some tools/users depend on the release assets being associated with the
permanent semantic tag (vX.Y.Z). So now we push assets to both.

https://github.com/neovim/neovim/issues/10011#issuecomment-492603135

neovim/bot-ci,neovim/bot-ci,neovim/bot-ci