CI: make sure the lockfile is up-to-date

Author: RalfJung

ci/script.sh

@@ -3,7 +3,7 @@ set -euxo pipefail
 beginswith() { case $2 in "$1"*) true;; *) false;; esac; }
 
 main() {
-    cross build --target $TARGET
+    cross build --target $TARGET --locked
     cross run --target $TARGET -- -V
 
     if beginswith nightly $TRAVIS_RUST_VERSION; then