scylla: add scylla_cloud_tests to cfg allowlist

With rust 1.80, cfg parameters are checked by the compiler, so that
warnings are emitted when unexpected cfg parameter is encountered.
As we are using `cfg(scylla_cloud_tests)` to conditionally compile tests
applicable for Scylla Serverless Cloud, that parameter is listed in
Cargo.toml to prevent warnings.

scylla/Cargo.toml

@@ -96,3 +96,4 @@ harness = false
 
 [lints.rust]
 unreachable_pub = "warn"
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(scylla_cloud_tests)'] }