config_paths: fix compiled-in locale dir

Fish uses this logic to find paths to functions etc.:

1. if $(status fish-path) starts with $CARGO_MANIFEST_DIR,
   we use $CARGO_MANIFEST_DIR/share etc.

   Aside: this also has the unintended effect that "cmake
   -DCMAKE_INSTALL_PREFIX=$PWD/prefix" will not use "$PWD/prefix/share" but
   "$PWD/share", at least since eacbd6156d (Port and adopt main written in
   Rust, 2023-08-18).

2. Else if $(status fish-path) ends with "bin/fish",
   and $(status fish-path)/../share/fish exists, we use that, since 4912967eab
   (Large set of changes related to making fish relocatable, and improving
   the build and install story, 2012-07-08)

3. Else if $(status fish-path) ends in "fish" (which is very likely),
   and $(status fish-path)/share exists, we use that, since
   c2a8de4873 (Make fish find config directories in source tree, 2016-09-23).
   I think this is for running (without installing) in-tree builds ("cmake .");
   this is not recommended but it is used, see
   https://github.com/fish-shell/fish-shell/pull/10330

4. If none of the above worked, either because the fish binary has been
   moved into a weird directory, or if we fail to get $(status fish-path)
   (e.g. on OpenBSD, where "argv[0]" is not available),
   then we fall back to reasonable default paths determined at compiled
   time.

   These paths include data_dir=$PREFIX/share.
   We recently added locale_dir too in bf65b9e3a7 (Change `gettext` paths
   to be relocatable (#11195), 2025-03-30).

In case 1, we use               locale: manifest_dir.join("share/locale"),
In case 2 and 3, we use         locale: data_dir.join("locale"),
In case 4, we use               locale: data_dir.join("share"),

The last one seems wrong (there is not "/usr/share/share"). Fix that.

Alternatively, we could revert bf65b9e3a7 (and redo the parts we want to keep).

src/env/config_paths.rs

@@ -7,6 +7,7 @@
 const DATA_DIR: &str = env!("DATADIR");
 const DATA_DIR_SUBDIR: &str = env!("DATADIR_SUBDIR");
 const SYSCONF_DIR: &str = env!("SYSCONFDIR");
+const LOCALE_DIR: &str = env!("LOCALEDIR");
 const BIN_DIR: &str = env!("BINDIR");
 
 pub static CONFIG_PATHS: Lazy<ConfigPaths> = Lazy::new(|| {
@@ -108,6 +109,11 @@
         } else {
             Some(PathBuf::from(BIN_DIR))
         };
+        let locale = if cfg!(feature = "embed-data") {
+            None
+        } else {
+            Some(PathBuf::from(LOCALE_DIR))
+        };
 
         FLOG!(config, "Using compiled in paths:");
         paths = ConfigPaths {
@@ -115,7 +121,7 @@
             sysconf: PathBuf::from(SYSCONF_DIR).join("fish"),
             doc: DOC_DIR.into(),
             bin,
-            locale: data.map(|x| x.join("share")),
+            locale,
         }
     }