fix: Add missing return in main (#11786)

AztecProtocol/aztec-packages#10217 introduced a
bug where the verifier result would not be sent from bb main as a
signal. This triggers a mysterious error in the ivc-integration tests
suite, native only, but does not break any e2e tests. More investigation
is necessary, and ultimately this should be resolved in the ACIR tests,
but it feels prudent now to fix the bug and reinstate the tests later.

cpp/src/barretenberg/bb/main.cpp

@@ -1409,7 +1409,7 @@ int main(int argc, char* argv[])
             if (command == "write_arbitrary_valid_proof_and_vk_to_file") {
                 const std::filesystem::path output_dir = get_option(args, "-o", "./target");
                 api.write_arbitrary_valid_proof_and_vk_to_file(flags, output_dir);
-                return 1;
+                return 0;
             }
 
             throw_or_abort("Invalid command passed to execute_command in bb");
@@ -1424,7 +1424,7 @@ int main(int argc, char* argv[])
 
         if (proof_system == "client_ivc") {
             ClientIVCAPI api;
-            execute_command(command, flags, api);
+            return execute_command(command, flags, api);
         } else if (command == "prove_and_verify") {
             return proveAndVerify(bytecode_path, recursive, witness_path) ? 0 : 1;
         } else if (command == "prove_and_verify_ultra_honk") {