properly assigns the custom shell

pkgs/fish-rust/default.nix

@@ -1,13 +1,16 @@
-{ lib, rustPlatform, fetchgit, ncurses, sphinx, python3 }:
+{ lib, rustPlatform, fetchgit, ncurses, python3Packages, gettext }:
+
 rustPlatform.buildRustPackage rec {
   pname = "fish";
   version = "4.1-2025-03-14-rust";
+
   src = fetchgit {
     url = "https://github.com/fish-shell/fish-shell.git";
     rev = "refs/heads/master";
     fetchSubmodules = true;
     sha256 = "18zpwa3yddic6wdwj7g51w6n4apwsixfvl179yddk2nwfpxhv4hq";
   };
+
   cargoLock = {
     lockFile = "${src}/Cargo.lock";
     outputHashes = {
@@ -15,26 +18,41 @@ rustPlatform.buildRustPackage rec {
       "pcre2-sys-0.2.9" = "0mhjw7fvrzxb3fd0c534a17qgy6svz0z8269d2fs6q8aw11610mr";
     };
   };
-  buildInputs = [ ncurses ];
-  nativeBuildInputs = [ sphinx python3 ];
+
+  buildInputs = [ ncurses gettext ];
+  nativeBuildInputs = with python3Packages; [
+    sphinx
+    sphinx_rtd_theme
+    gettext
+  ];
+
   preBuild = ''
     export FISH_BUILD_VERSION="${version}"
     export FISH_BUILD_DOCS=1
   '';
+
   postInstall = ''
     export HOME=$(mktemp -d)
     mkdir -p $HOME/.local/share/fish/install
-    $out/bin/fish --install
+
+    if [ ! -f $HOME/.config/fish/config.fish ]; then
+      $out/bin/fish --install
+    fi
+
     mkdir -p $out/share/fish
     cp -r $HOME/.local/share/fish/install/* $out/share/fish/ 2>/dev/null || true
+
     mkdir -p $out/share/fish/tools
     cp $src/share/tools/create_manpage_completions.py $out/share/fish/tools/
     cp $src/share/tools/deroff.py $out/share/fish/tools/
     chmod +x $out/share/fish/tools/create_manpage_completions.py
     chmod +x $out/share/fish/tools/deroff.py
+
     rm -rf $HOME
   '';
+
   doCheck = false;
+
   meta = with lib; {
     description = "The user-friendly command line shell (Rust version)";
     homepage = "https://fishshell.com/";
@@ -42,5 +60,6 @@ rustPlatform.buildRustPackage rec {
     platforms = platforms.unix;
     mainProgram = "fish";
   };
+
   passthru.shellPath = "/bin/fish";
 }