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

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

** building package indices
** testing if installed package can be loaded
* DONE (igraph)
post-installation fixup
shrinking RPATHs of ELF executables and libraries in /nix/store/fiaqq2hyha39w50dmrjqmayqiwnwd6rp-r-igraph-1.2.1
shrinking /nix/store/fiaqq2hyha39w50dmrjqmayqiwnwd6rp-r-igraph-1.2.1/library/igraph/libs/igraph.so
strip is /nix/store/4mf2xm9p32lzrim927yk92xhx35yaz62-binutils-2.30/bin/strip
patching script interpreter paths in /nix/store/fiaqq2hyha39w50dmrjqmayqiwnwd6rp-r-igraph-1.2.1
checking for references to /build in /nix/store/fiaqq2hyha39w50dmrjqmayqiwnwd6rp-r-igraph-1.2.1...
/nix/store/fiaqq2hyha39w50dmrjqmayqiwnwd6rp-r-igraph-1.2.1

** byte-compile and prepare package for lazy loading
** help
*** installing help indices
** building package indices
** testing if installed package can be loaded
* DONE (igraph)
post-installation fixup
strip is /nix/store/8q05v40n0f7amgw70pk6bkb23r7hrzgn-cctools-binutils-darwin/bin/strip
patching script interpreter paths in /nix/store/jns9nscb3m2hmaq0h9bbm2nx9wd2rmab-r-igraph-1.2.1
/nix/store/jns9nscb3m2hmaq0h9bbm2nx9wd2rmab-r-igraph-1.2.1