Revert "libcosmicAppHook: use RUSTFLAGS instead of platform specific flags (NixOS#464981)"

This reverts commit d2c6221.

PR NixOS#464981 changed libcosmicAppHook to use the global RUSTFLAGS instead
of target-specific CARGO_TARGET_*_RUSTFLAGS as a workaround for issue
PR NixOS#464392. However, this broke cross-compilation for packages using the
crabtime procedural macro (like cosmic-initial-setup).

The root cause was that buildRustPackage was always setting env.RUSTFLAGS,
which shadowed the target-specific flags. This has been fixed in commit
PR NixOS#464707, which only sets RUSTFLAGS when explicitly
needed.

Fixes cross-compilation of cosmic-initial-setup and other cosmic
packages that use crabtime proc macros.

Signed-off-by: Brian McGillion <bmg.avoin@gmail.com>

Author: brianmcgillion

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

@@ -57,10 +57,7 @@ makeSetupHook {
       lib.makeSearchPath "share" (
         lib.optionals includeSettings [ fallbackThemes ] ++ [ targetPackages.cosmic-icons or 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 (
       [