Force libc wcwidth to use UTF-8 locale again

Commit 046db09f90 (Try to set LC_CTYPE to something UTF-8 capable
(#8031), 2021-06-06) forced UTF-8 encoding if available.

Since c8001b5023 (encoding: use UTF-8 everywhere, 2025-10-18) we no
longer override LC_CTYPE, since we no longer use it for anything like
mbrtowc or iswalpha.

However there is one remaining use: our fallbacks to system wcwidth().
If we are started as

	LC_ALL=C fish

then wcwidth('😃') will fail to return the correct value, even if
the UTF-8 locale would have been available.

Restore the previous behavior, so locale variables don't affect
emoji width.

This is consistent with the direction in c8001b5023 which stops us
from falling back to ASCII characters if our desired multibyte locale
is missing (that was not the ideal criteria).

In future we should maybe stop using wcwidth().

src/locale.rs

@@ -9,7 +9,9 @@
 /// Call this either before starting any locale-using thread, or while holding a lock on the
 /// above mutex.
 pub unsafe fn set_libc_locales() -> Option<&'static CStr> {
-    setlocale(libc::LC_ALL, Some(c""))
+    let opaque_locale_str = setlocale(libc::LC_ALL, Some(c""));
+    setlocale(libc::LC_CTYPE, Some(c"C.UTF-8"));
+    opaque_locale_str
 }
 
 fn setlocale(category: libc::c_int, locale: Option<&CStr>) -> Option<&'static CStr> {