mingetty: fix cross compilation

pkgs/os-specific/linux/mingetty/default.nix

···
     sha256 = "05yxrp44ky2kg6qknk1ih0kvwkgbn9fbz77r3vci7agslh5wjm8g";
   };
 
+  makeFlags = [
+    "CC:=$(CC)"
+    "SBINDIR=${placeholder "out"}/sbin"
+    "MANDIR=${placeholder "out"}/share/man/man8"
+  ];
+
   preInstall = ''
     mkdir -p $out/sbin $out/share/man/man8
-    makeFlagsArray=(SBINDIR=$out/sbin MANDIR=$out/share/man/man8)
   '';
 
   meta = with lib; {