Occasionally, we may need to silence warnings and clippy lints that
were only introduced in newer Rust compiler versions. However, this
would fail when compiling with an older rustc:
error: unknown lint: `non_local_definitions`
--> rust/qemu-api/rust-qemu-api-tests.p/structured/offset_of.rs:79:17
So by default we need to block the unknown_lints warning. To avoid
misspelled lints or other similar issues, re-enable it in the CI job
that uses nightly rust.
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>