busybox-sandbox-shell: fix eval for musl (#361265)

pkgs/os-specific/linux/busybox/sandbox-shell.nix

@@ -1,12 +1,10 @@
 { lib, stdenv, busybox, musl }:
 
 # Minimal shell for use as basic /bin/sh in sandbox builds
-busybox.override {
+busybox.override ({
   enableStatic = true;
   enableMinimal = true;
 
-  useMusl = stdenv.hostPlatform.isGnu && lib.meta.availableOn stdenv.hostPlatform musl;
-
   extraConfig = ''
     CONFIG_FEATURE_FANCY_ECHO y
     CONFIG_FEATURE_SH_MATH y
@@ -26,4 +24,6 @@ busybox.override {
     CONFIG_ASH_PRINTF y
     CONFIG_ASH_TEST y
   '';
-}
+} // lib.optionalAttrs (stdenv.hostPlatform.isGnu && lib.meta.availableOn stdenv.hostPlatform musl) {
+  useMusl = true;
+})