Pin CBMC version to 5.81.0 (#1022)

This is due to a failure of aws_hash_table_clean_up with 5.82.

Author: karkhaz

.github/workflows/proof_ci_resources/config.yaml

@@ -1,5 +1,5 @@
 cadical-tag: latest
-cbmc-version: latest
+cbmc-version: "5.81.0"
 cbmc-viewer-version: latest
 kissat-tag: latest
 litani-version: latest