%s -> %ls for wchar_t

I noticed my debug output for 24bit color mode was garbled due to
this being wrong. I spent a little time trying to get the compiler
to tell us about these, but -Wformat doesn't do anything for wchar
printf functions, and __attribute__((format(printf, n, m))) will
cause an error with wchar_t's, so I gave up and decided to manually
check out every '%s' in the entire project. I found (only) one
more.

debug(0, "%s", wchars) will report warnings for incorrect
specifiers but debug(0, L"%s", wchars) is unable. Thus there may
be reason to prefer not using L"..." as an argument if all else
is equal and it's not necessary.

src/env_dispatch.cpp

@@ -376,7 +376,7 @@ static void update_fish_color_support(const environment_t &vars) {
     // Handle $fish_term24bit
     if (auto fish_term24bit = vars.get(L"fish_term24bit")) {
         support_term24bit = bool_from_string(fish_term24bit->as_string());
-        debug(2, L"'fish_term24bit' preference: 24-bit color %s",
+        debug(2, L"'fish_term24bit' preference: 24-bit color %ls",
               support_term24bit ? L"enabled" : L"disabled");
     } else {
         // We don't attempt to infer term24 bit support yet.