gmic: fix cross build (#375571)

pkgs/by-name/gm/gmic/package.nix

···
   stdenv,
   fetchFromGitHub,
   fetchurl,
+  buildPackages,
   cimg,
   cmake,
   common-updater-scripts,
···
     + lib.optionalString stdenv.hostPlatform.isDarwin ''
       substituteInPlace CMakeLists.txt \
         --replace "LD_LIBRARY_PATH" "DYLD_LIBRARY_PATH"
+    ''
+    + lib.optionalString (!stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
+      substituteInPlace CMakeLists.txt --replace-fail \
+        'LD_LIBRARY_PATH=''${GMIC_BINARIES_PATH} ''${GMIC_BINARIES_PATH}/gmic' \
+        '${lib.getExe buildPackages.gmic}'
     '';
 
   passthru = {