Revert "libcosmicAppHook: use RUSTFLAGS instead of platform specific flags"

This reverts commit d2c62217d3.

This is no longer needed in staging due to the proper fix https://github.com/NixOS/nixpkgs/pull/464707 having been merged

pkgs/by-name/li/libcosmicAppHook/libcosmic-app-hook.sh

@@ -15,11 +15,7 @@ libcosmicAppVergenHook() {
 
 libcosmicAppLinkerArgsHook() {
   # Force linking to certain libraries like libEGL, which are always dlopen()ed
-  # local flags="CARGO_TARGET_@cargoLinkerVar@_RUSTFLAGS"
-
-  # Temporarily use this simpler solution, it should work for simple cross compilation
-  # https://github.com/NixOS/nixpkgs/issues/464392
-  local flags="RUSTFLAGS"
+  local flags="CARGO_TARGET_@cargoLinkerVar@_RUSTFLAGS"
 
   export "$flags"="${!flags-} -C link-arg=-Wl,--push-state,--no-as-needed"
   # shellcheck disable=SC2043

pkgs/by-name/li/libcosmicAppHook/package.nix

@@ -55,10 +55,7 @@ makeSetupHook {
       lib.makeSearchPath "share" (
         lib.optionals includeSettings [ fallbackThemes ] ++ [ targetPackages.cosmic-icons ]
       );
-    # Temporarily using RUSTFLAGS: https://github.com/NixOS/nixpkgs/issues/464392
-    # See ./libcosmic-app-hook.sh
-    # cargoLinkerVar = targetPackages.stdenv.hostPlatform.rust.cargoEnvVarTarget;
-
+    cargoLinkerVar = targetPackages.stdenv.hostPlatform.rust.cargoEnvVarTarget;
     # force linking for all libraries that may be dlopen'd by libcosmic/iced apps
     cargoLinkLibs = lib.escapeShellArgs (
       [