`nix eval` forces values and prints derivations as attribute sets, so
commands that print derivations (e.g. `nix eval nixpkgs#bash`) will
infinitely loop and segfault.
Printing derivations as `.drv` paths makes `nix eval` complete as
expected. Further work is needed, but this is better than a segfault.
(cherry picked from commit 4910d74086a85876e093136a0e8ebc547b467af7)
Change-Id: I8e1cb39c05db812080759ec183ee7a131760e6ea
Rebecca Turner
6ef9d8efba