Nightly as-if-std is failing on some targets with unused import `std::*`

[Chris Denton]

Specifically ubuntu-20.04, nightly, macos-latest, nightly and x86_64-fortanix-unknown-sgx. For example:

Run cargo build --manifest-path crates/as-if-std/Cargo.toml --no-default-features
  
   Compiling as-if-std v0.1.0 (/Users/runner/work/backtrace-rs/backtrace-rs/crates/as-if-std)
error: unused import: `std::*`
  --> crates/as-if-std/src/lib.rs:14:13
   |
14 |     pub use std::*;
   |             ^^^^^^
   |
   = note: `-D unused-imports` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(unused_imports)]`

error: could not compile `as-if-std` (lib) due to previous error
Error: Process completed with exit code 101.

Apparently the #[allow(unused_imports)] on the re-export is no longer enough to satisfy it.