test: clear parent envs to prevent leakage to tests (#9976)

Running tests locally from nushell with customizations (i.e.
$env.PROMPT_COMMAND etc) may lead to failing tests as that customization
leaks to the sandboxed nu itself.

Remove `FILE_PWD` from env

# Tests + Formatting

Tests are now passing locally without issue in my case

crates/nu-protocol/src/shell_error.rs

@@ -431,7 +431,7 @@ pub enum ShellError {
     #[diagnostic(
         code(nu::shell::automatic_env_var_set_manually),
         help(
-            r#"The environment variable '{envvar_name}' is set automatically by Nushell and cannot not be set manually."#
+            r#"The environment variable '{envvar_name}' is set automatically by Nushell and cannot be set manually."#
         )
     )]
     AutomaticEnvVarSetManually {