tools/install.sh: soft requirement on nix

tools/install.sh

@@ -9,5 +9,11 @@ cd "${hscloud_root}"
 
 bazel build \
         //tools:kubectl //tools:kubecfg //tools:secretstore \
-        //tools:pass //tools:calicoctl.bin //tools:cfssl \
-        //tools:nixops
+        //tools:pass //tools:calicoctl.bin //tools:cfssl
+
+if [ ! -e /nix ] ; then
+    echo "WARNING: No Nix installation detected. nix-dependent tools (nixops) will not be built or available." 
+else
+    bazel build \
+            //tools:nixops
+fi