kernel: Respect makeFlagsArray in more places

It's used two lines above, so be consistent.

pkgs/os-specific/linux/kernel/manual-config.nix

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         make $makeFlags "''${makeFlagsArray[@]}" oldconfig
         runHook postConfigure
 
-        make $makeFlags prepare
+        make $makeFlags "''${makeFlagsArray[@]}" prepare
         actualModDirVersion="$(cat $buildRoot/include/config/kernel.release)"
         if [ "$actualModDirVersion" != "${modDirVersion}" ]; then
           echo "Error: modDirVersion ${modDirVersion} specified in the Nix expression is wrong, it should be: $actualModDirVersion"