a) For `nixos-rebuild` you can set
  { nixpkgs.config.allowUnsupportedSystem = true; }
in configuration.nix to override this.

b) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
  { allowUnsupportedSystem = true; }
to ~/.config/nixpkgs/config.nix.

  CC       nouveau_span.lo
  CC       nouveau_state.lo
  CC       nouveau_surface.lo
  CC       nouveau_texture.lo
  CC       nv04_context.lo
  CC       nv04_render.lo
  CC       nv04_state_fb.lo
  CC       nv04_state_frag.lo
building of '/nix/store/mkgvlja2if9sl1vk1dy5xr487va3yj5i-mesa-noglu-18.1.4.drv' timed out after 3600 seconds
error: build of '/nix/store/mkgvlja2if9sl1vk1dy5xr487va3yj5i-mesa-noglu-18.1.4.drv' failed

stripping (with command strip and flags -S) in /nix/store/l038l6s0c7k94mqbyzyhjh5mg32kxsid-mesa-noglu-18.1.4-drivers/lib
patching script interpreter paths in /nix/store/l038l6s0c7k94mqbyzyhjh5mg32kxsid-mesa-noglu-18.1.4-drivers
checking for references to /build in /nix/store/l038l6s0c7k94mqbyzyhjh5mg32kxsid-mesa-noglu-18.1.4-drivers...
shrinking RPATHs of ELF executables and libraries in /nix/store/dcg6qa8x5dk8hqnajpf0d4idyamiwk1p-mesa-noglu-18.1.4-osmesa
shrinking /nix/store/dcg6qa8x5dk8hqnajpf0d4idyamiwk1p-mesa-noglu-18.1.4-osmesa/lib/libOSMesa.so.8.0.0
strip is /nix/store/h0nc84ybvb8g724fwysvcghh1675wjli-binutils-2.30/bin/strip
stripping (with command strip and flags -S) in /nix/store/dcg6qa8x5dk8hqnajpf0d4idyamiwk1p-mesa-noglu-18.1.4-osmesa/lib
patching script interpreter paths in /nix/store/dcg6qa8x5dk8hqnajpf0d4idyamiwk1p-mesa-noglu-18.1.4-osmesa
checking for references to /build in /nix/store/dcg6qa8x5dk8hqnajpf0d4idyamiwk1p-mesa-noglu-18.1.4-osmesa...
/nix/store/l038l6s0c7k94mqbyzyhjh5mg32kxsid-mesa-noglu-18.1.4-drivers