mesa WIP: adding salt to restart hydra

Hydra seems to have a problem with multiple-output derivations.

pkgs/development/libraries/mesa/default.nix

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   # move gallium-related stuff to $drivers, so $out doesn't depend on LLVM
   # ToDo: probably not all .la files are completely fixed, but it shouldn't matter
   postInstall = ''
+      # salt to restart hydra
     mv -t "$drivers/lib/" \
       $out/lib/libdricore* \
       $out/lib/libgallium.* \