use plugin add instead of nu -c "register ..." (#69)

as per title

build.nu

@@ -8,7 +8,7 @@ def main [package_file: path] {
     let name = open ($repo_root | path join "Cargo.toml") | get package.name
 
     cargo install --path $repo_root --root $install_root
-    ^$nu.current-exe --commands $"register ($install_root | path join "bin" $name)"
+    plugin add ($install_root | path join "bin" $name)
 
     log info "do not forget to restart Nushell for the plugin to be fully available!"
 }