Respect system locale when formatting file sizes via config (#15271)

# Description

Commands and other pieces of code using `$env.config.format.filesize` to
format filesizes now respect the system locale when formatting the
numeric portion of a file size.

# User-Facing Changes

- System locale is respected when using `$env.config.format.filesize` to
format file sizes.
- Formatting a file size with a binary unit is now exact for large file
sizes and units.
- The output of `to text` is no longer dependent on the config.

crates/nu-utils/src/locale.rs

@@ -22,9 +22,16 @@ pub fn get_system_locale() -> Locale {
 
 #[cfg(debug_assertions)]
 pub fn get_system_locale_string() -> Option<String> {
-    std::env::var(LOCALE_OVERRIDE_ENV_VAR)
-        .ok()
-        .or_else(sys_locale::get_locale)
+    std::env::var(LOCALE_OVERRIDE_ENV_VAR).ok().or_else(
+        #[cfg(not(test))]
+        {
+            sys_locale::get_locale
+        },
+        #[cfg(test)]
+        {
+            || Some(Locale::en_US_POSIX.name().to_owned())
+        },
+    )
 }
 
 #[cfg(not(debug_assertions))]