nextpnr: fix build on macOS
nextpnr: fix build on macOS

`PYTRELLIS_LIBDIR` has been replaced by `TRELLIS_LIBDIR` in https://github.com/YosysHQ/nextpnr/commit/1dc1164dce88fade762956c1067aeb97fa3c0f9a, which causes the build to fail on macOS.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs