x64 new-backend CI: use Cargo.lock in build.

We do a `cargo fetch --locked` for most of our CI builds, but
`run-experimental-x64-ci.sh` was not doing this. As a result, some CI
runs seem to fail depending on which versions of crates they download. A
common failure mode is that two different versions of the `syn` crate
get into the build somehow, resulting in errors in wiggle/witx.

This change simply adds the `--locked` flag to the `cargo test` run for
the new x64 backend.

ci/run-experimental-x64-ci.sh

@@ -6,6 +6,7 @@
 CARGO_VERSION=${CARGO_VERSION:-"+nightly"}
 
 cargo $CARGO_VERSION \
+            --locked \
             -Zfeatures=all -Zpackage-features \
             test \
             --features test-programs/test_programs \