qsynth: use qt5.mkDerivation
qsynth: use qt5.mkDerivation

Fixes

  $ qsynth
  qt.qpa.plugin: Could not find the Qt platform plugin "xcb" in ""
  This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

  Aborted (core dumped)

(Also, take qt5.* attrs as input instead of the whole qt5 attrset itself.)

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