stdenv: fix crossSystem localSystem comparison

this appears to fix the following issue for some reason

```
nix-repl> (import ./. { localSystem.system = "x86_64-linux"; crossSystem.system = "x86_64-linux"; }).bash
«derivation /nix/store/5lz9p8xhf89kb1c1kk6jxrzskaiygnlh-bash-5.2-p15.drv»

nix-repl> (import ./. { localSystem.system = "x86_64-linux"; crossSystem = "x86_64-linux"; }).bash
«derivation /nix/store/1whiq03rsfrmch2ds3jhyqjaczfz97lb-bash-5.2-p15-x86_64-unknown-linux-gnu.drv»
```

fixed
```
nix-repl> (import ./. { localSystem.system = "x86_64-linux"; crossSystem = "x86_64-linux"; }).bash
«derivation /nix/store/5lz9p8xhf89kb1c1kk6jxrzskaiygnlh-bash-5.2-p15.drv»
```

pkgs/stdenv/default.nix

@@ -36,7 +36,7 @@ let
 
   # Select the appropriate stages for the platform `system'.
 in
-  if crossSystem != localSystem || crossOverlays != [] then stagesCross
+  if !lib.systems.equals crossSystem localSystem || crossOverlays != [] then stagesCross
   else if config ? replaceStdenv then stagesCustom
   else if localSystem.isLinux then stagesLinux
   else if localSystem.isDarwin then stagesDarwin

pkgs/stdenv/linux/default.nix

@@ -95,7 +95,7 @@
   in files
 }:
 
-assert crossSystem == localSystem;
+assert lib.systems.equals crossSystem localSystem;
 
 let
   inherit (localSystem) system;