nextpnr: restrict to Linux only

a5ac052d changed the platforms for
nextpnr from 'linux' to 'all', but unfortunately nextpnr has not been
building anyway due to a missing OpenGL dependency.[1][2]

Restrict back to Linux for now. When a build fix happens, we can open it
up again.

  [1] https://hydra.nixos.org/build/99106360
  [1] https://hydra.nixos.org/eval/1538169



Signed-off-by: Austin Seipp <aseipp@pobox.com>