wrapNeovimUnstable --run wrapperArgs execute at build time

Replacing a writeShellApplication nvim launcher with wrapNeovimUnstable wrapperArgs (--suffix PATH, --set-default, --run) is much smaller, and nix run works without an apps output. But the wrapper runs the built nvim once during the build to generate rplugin.vim, so any --run snippet executes in the sandbox too: a cp into $XDG_CONFIG_HOME/nvim failed with cannot create directory ... No such file or directory and Generating rplugin.vim failed!. Make --run snippets safe in a sandbox, e.g. mkdir -p first.