Update default_env.nu to work with windows (#9172)

# Description
This PR fixes the `create_left_prompt` custom command to work with
Windows.
Before:

![image](https://github.com/nushell/nushell/assets/343840/5a78fc3b-3c2a-4c2f-9c1d-2451063273af)
After:

![image](https://github.com/nushell/nushell/assets/343840/c3785767-5000-454e-9b7b-b0094c3d0834)


# User-Facing Changes
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# Tests + Formatting
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crates/nu-utils/src/sample_config/default_env.nu

@@ -21,7 +21,7 @@ def create_left_prompt [] {
     let separator_color = (if (is-admin) { ansi light_red_bold } else { ansi light_green_bold })
     let path_segment = $"($path_color)($dir)"
 
-    $path_segment | str replace --all (char path_sep) $"($separator_color)/($path_color)"
+    $path_segment | str replace --all --string (char path_sep) $"($separator_color)/($path_color)"
 }
 
 def create_right_prompt [] {