Fix wrong prompt in Nushell activation script (#2481)

Fixes #2480

Author: kubouch

docs/changelog/2481.bugfix.rst

@@ -0,0 +1 @@
+Fix broken prompt in Nushell when activating virtual environment.

src/virtualenv/activation/nushell/activate.nu

@@ -92,7 +92,7 @@ export-env {
 
       # If there is no default prompt, then only the env is printed in the prompt
       let new_prompt = if (has-env 'PROMPT_COMMAND') {
-          if ($old_prompt_command | describe) == 'block' {
+          if 'closure' in ($old_prompt_command | describe) {
               { $'($virtual_prompt)(do $old_prompt_command)' }
           } else {
               { $'($virtual_prompt)($old_prompt_command)' }