nixops: fix

Change-Id: I15ebde319fcae3f9771da6a549e52783e0ec4409
diff --git a/cluster/tools/nixops.sh b/cluster/tools/nixops.sh
index f3848d2..e810972 100755
--- a/cluster/tools/nixops.sh
+++ b/cluster/tools/nixops.sh
@@ -19,4 +19,4 @@
 export GCE_SERVICE_ACCOUNT="nixops@hscloud.iam.gserviceaccount.com"
 export ACCESS_KEYPATH="$hscloud_root/gcp/secrets/plain/sa.pem"
 
-nixops.bin "$@"
+./external/nixops/bin/nixops "$@"