gnuradio: wrapProgram makes gnuradio python module appear in PYTHONPATH

TODO: wrap all programs, not just uhd_fft.

TODO: get rid of hardcoded PYTHONPATH for gnuradio itself
($out/lib/python2.7/site-packages).

TODO: I still get errors at runtime. The rest of this message is error
from uhd_fft

  linux; GNU C++ version 4.6.3; Boost_105300; UHD_003.005.002-0-unknown

  Gtk-Message: Failed to load module "canberra-gtk-module"
  -- Loading firmware image:
  /nix/store/3c82pa85zcl5vr918nc645gna703vcx3-uhd-3.5.2/share/uhd/images/usrp1_fw.ihx...
  done
  -- Opening a USRP1 device...
  -- Loading FPGA image:
  /nix/store/3c82pa85zcl5vr918nc645gna703vcx3-uhd-3.5.2/share/uhd/images/usrp1_fpga.rbf...
  done
  -- Using FPGA clock rate of 64.000000MHz...
  Using Volk machine: avx_64_mmx_orc
  Traceback (most recent call last):
    File
  "/nix/store/n3mb63f1cqk4gv5w75w5nf4s0fvcs1c2-gnuradio-3.6.4.1/bin/.uhd_fft-wrapped",
  line 341, in <module>
      main ()
    File
  "/nix/store/n3mb63f1cqk4gv5w75w5nf4s0fvcs1c2-gnuradio-3.6.4.1/bin/.uhd_fft-wrapped",
  line 337, in main
      app = stdgui2.stdapp(app_top_block, "UHD FFT", nstatus=1)
    File
  "/nix/store/n3mb63f1cqk4gv5w75w5nf4s0fvcs1c2-gnuradio-3.6.4.1/lib/python2.7/site-packages/gnuradio/wxgui/stdgui2.py",
  line 38, in __init__
      wx.App.__init__ (self, redirect=False)
    File
  "/nix/store/xvchl7cy1f9cpyv71l17dpyxm8h93pvr-wxPython-2.8.12.1/lib/python2.7/site-packages/wx/_core.py",
  line 7981, in __init__
      self._BootstrapApp()
    File
  "/nix/store/xvchl7cy1f9cpyv71l17dpyxm8h93pvr-wxPython-2.8.12.1/lib/python2.7/site-packages/wx/_core.py",
  line 7555, in _BootstrapApp
      return _core_.PyApp__BootstrapApp(*args, **kwargs)
    File
  "/nix/store/n3mb63f1cqk4gv5w75w5nf4s0fvcs1c2-gnuradio-3.6.4.1/lib/python2.7/site-packages/gnuradio/wxgui/stdgui2.py",
  line 42, in OnInit
      self._max_noutput_items)
    File
  "/nix/store/n3mb63f1cqk4gv5w75w5nf4s0fvcs1c2-gnuradio-3.6.4.1/lib/python2.7/site-packages/gnuradio/wxgui/stdgui2.py",
  line 64, in __init__
      self.panel = stdpanel (self, self, top_block_maker, max_nouts)
    File
  "/nix/store/n3mb63f1cqk4gv5w75w5nf4s0fvcs1c2-gnuradio-3.6.4.1/lib/python2.7/site-packages/gnuradio/wxgui/stdgui2.py",
  line 86, in __init__
      self.top_block = top_block_maker (frame, self, vbox, sys.argv)
    File
  "/nix/store/n3mb63f1cqk4gv5w75w5nf4s0fvcs1c2-gnuradio-3.6.4.1/bin/.uhd_fft-wrapped",
  line 125, in __init__
      self.scope.set_callback(fftsink_callback)
    File
  "/nix/store/n3mb63f1cqk4gv5w75w5nf4s0fvcs1c2-gnuradio-3.6.4.1/lib/python2.7/site-packages/gnuradio/gr/hier_block2.py",
  line 54, in __getattr__
      return getattr(self._hb, name)
  AttributeError: 'gr_hier_block2_sptr' object has no attribute
  'set_callback'