Ignore cache-poisoning lint for cargo-vet.yml

This lint is primarily for publish workflows that could use a poisoned
cache for release artifacts, but our cargo-vet workflow isn't that sensitive
so keeping the caching is worth it.

Author: legoktm

.github/zizmor.yml

@@ -0,0 +1,5 @@
+rules:
+  cache-poisoning:
+    ignore:
+      # not used for releases / sensitive artifacts
+      - cargo-vet.yml