Nixops requires nix_rules, which in turn requires a working nix
installation.
When we split tools/install.sh into tools/install.sh and
cluster/tools/install.sh [1], we accidentally made the latter always install
all cluster tools, including nixops - even if the install.sh script
detected that the system does not have Nix installed.
[1] - https://gerrit.hackerspace.pl/c/hscloud/+/81
Change-Id: Ib5357cfe125f1393b395b28062787f3f0091f549
