fcitx5-rime: add an environment variable for shared data dir passing librime.

The original RIME_DATA_DIR is fixed at build time which is set to the package
dir under nix store. A such dir is read only for users so if some one want to
add a shared data, he has to override fcitx5-rime or librime, which isn't
user-friendly.

On other distributions, RIME_DATA_DIR may be under /usr/local/share/fcitx5 which
is not existed on NixOS. If we set its to /run/current-system/sw/share/..., this
will break for non-NixOS user.

Therefore, we patch it with a special environment variable to let user to load
it dynamically.

Co-Authored-by: Lan Tian <xuyh0120@outlook.com>

pkgs/tools/inputmethods/fcitx5/fcitx5-rime-with-nix-env-variable.patch

@@ -0,0 +1,18 @@
+:100644 100644 fac4f53 aed9617 M	src/rimeengine.cpp
+
+diff --git a/src/rimeengine.cpp b/src/rimeengine.cpp
+index fac4f53..aed9617 100644
+--- a/src/rimeengine.cpp
++++ b/src/rimeengine.cpp
+@@ -164,7 +164,10 @@ void RimeEngine::rimeStart(bool fullcheck) {
+             RIME_ERROR() << "Failed to create user directory: " << userDir;
+         }
+     }
+-    const char *sharedDataDir = RIME_DATA_DIR;
++    const char *sharedDataDir = getenv("NIX_RIME_DATA_DIR");
++    if (!sharedDataDir) {
++        sharedDataDir = RIME_DATA_DIR;
++    }
+ 
+     RIME_STRUCT(RimeTraits, fcitx_rime_traits);
+     fcitx_rime_traits.shared_data_dir = sharedDataDir;

pkgs/tools/inputmethods/fcitx5/fcitx5-rime.nix

@@ -35,6 +35,8 @@ stdenv.mkDerivation rec {
     librime
   ];
 
+  patches = [ ./fcitx5-rime-with-nix-env-variable.patch ];
+
   meta = with lib; {
     description = "RIME support for Fcitx5";
     homepage = "https://github.com/fcitx/fcitx5-rime";