cluster/tools/install.sh: fix nixops graceful degradation

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

cluster/tools/install.sh

@@ -9,7 +9,11 @@ fi
 
 cd "${hscloud_root}"
 
-bazel build //cluster/tools/...
+bazel build \
+        //cluster/tools:kubectl \
+        //cluster/tools:kubecfg \
+        //cluster/tools:calicoctl \
+        //cluster/tools:cfssl
 
 if [ ! -e /nix ] ; then
     echo "WARNING: No Nix installation detected. nix-dependent tools (nixops) will not be built or available."