Temporarily silence a Rust error

The generated Rust code from our proto files triggers a warning in Rust
nightly, which we treat as an error:

    error: custom inner attributes are unstable
    Error:  --> src/../proto/metadata.rs:9:4
      |
    9 | #![rustfmt::skip]
      |    ^^^^^^^^^^^^^
      |
      = note: `#[deny(soft_unstable)]` on by default
      = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
      = note: for more information, see issue #64266 <rust-lang/rust#64266>

This is already reported in the `rust-protobuf` repo [1] so until it's
fixed, we choose to silence this warning to make our builds work.

[1]: stepancheg/rust-protobuf#551

src/lib.rs

@@ -39,6 +39,9 @@
     unused_results,
     variant_size_differences
 )]
+// FIXME: This must be removed once this is solved:
+// https://github.com/stepancheg/rust-protobuf/issues/551
+#![allow(soft_unstable)]
 
 pub mod aead;
 pub mod cryptors;