glmark2: don't overwrite LD_LIBRARY_PATH from environment

For driver development, I usually point LD_LIBRARY_PATH at an
install-dir from an in-progress mesa checkout. Clearing the previous
LD_LIBRARY_PATH value breaks this, and causes glmark2 to attempt to use
the system drivers.

pkgs/by-name/gl/glmark2/package.nix

@@ -55,7 +55,7 @@ stdenv.mkDerivation rec {
   postInstall = ''
     for binary in $out/bin/glmark2*; do
       wrapProgram $binary \
-        --set LD_LIBRARY_PATH ${libGL}/lib
+        --prefix LD_LIBRARY_PATH ":" ${libGL}/lib
     done
   '';