Fix mise shell setting env in nushell

Currently `mise shell` doesn't have an effect in latest nushell.
Functions in nushell that want to export environment variables
must use the `--env` flag in the function definition.

This flag was used on other functions in mise.nu as required
(eg. update-env), however it seems that this flag is also required
on the `main` function that manages updating the env for `mise shell`.

src/shell/nushell.rs

@@ -48,7 +48,7 @@ impl Shell for Nushell {
             $in | lines | parse "{{op}},{{name}},{{value}}"
           }}
 
-          export def --wrapped main [command?: string, --help, ...rest: string] {{
+          export def --env --wrapped main [command?: string, --help, ...rest: string] {{
             let commands = ["shell", "deactivate"]
 
             if ($command == null) {{

src/shell/snapshots/mise__shell__nushell__tests__hook_init.snap

@@ -22,7 +22,7 @@ def "parse vars" [] {
   $in | lines | parse "{op},{name},{value}"
 }
 
-export def --wrapped main [command?: string, --help, ...rest: string] {
+export def --env --wrapped main [command?: string, --help, ...rest: string] {
   let commands = ["shell", "deactivate"]
 
   if ($command == null) {