Fix VHS for Nushell version ^0.83 (#373)

* Changed nushell prompt command

Removed let-env in favor of $env. This supports nushell versions >= 0.83

* Update shell.go

shell.go

@@ -62,6 +62,6 @@ var Shells = map[string]Shell{
 		Command: []string{"cmd.exe", "/k", "prompt=^> "},
 	},
 	nushell: {
-		Command: []string{"nu", "--interactive", "--execute", "let-env PROMPT_COMMAND = { '' }"},
+		Command: []string{"nu", "--execute", "$env.PROMPT_COMMAND = {''}"},
 	},
 }