installer tests: Use different package for testing `nix-env -iA`

Coreutils is multi-output and the `info` output doesn't seem to be
included on the install disk, failing like this (because now nix-env
wants to build coreutils):

````
machine# these derivations will be built:
machine#   /nix/store/0jk4wzg11sa6cqyw8g7w5lb35axji969-bison-3.0.4.tar.gz.drv
...
machine#   /nix/store/ybjgqwxx63l8cj1s7b8axx09wz06kxbv-coreutils-8.25.drv
machine# building path(s) ‘/nix/store/4xvdi5740vq8vlsi48lik3saz0v5jsx0-coreutils-8.25.tar.xz’
machine# downloading ‘http://ftpmirror.gnu.org/coreutils/coreutils-8.25.tar.xz’...
machine# error: unable to download ‘http://ftpmirror.gnu.org/coreutils/coreutils-8.25.tar.xz’: Couldn't resolve host name (6)
machine# builder for ‘/nix/store/5j3bc5sjr6271fnjh9gk9hrid8kgbpx3-coreutils-8.25.tar.xz.drv’ failed with exit code 1
machine# cannot build derivation ‘/nix/store/ybjgqwxx63l8cj1s7b8axx09wz06kxbv-coreutils-8.25.drv’: 1 dependencies couldn't be built
machine# error: build of ‘/nix/store/ybjgqwxx63l8cj1s7b8axx09wz06kxbv-coreutils-8.25.drv’ failed
````