Bug 1773592 - Pass --frozen to cargo-vet in CI. r=glandium

This is the new flag to ensure we don't hit the network.

Differential Revision: https://phabricator.services.mozilla.com/D148869

python/mozbuild/mozbuild/vendor/vendor_rust.py

@@ -749,6 +749,7 @@ license file's hash.
         flags = ["--output-format=json"]
         if "MOZ_AUTOMATION" in os.environ:
             flags.append("--locked")
+            flags.append("--frozen")
         res = cargo_vet(
             self,
             flags,