make oss fails on stable Rust: error[E0554]: #![feature] may not be used on stable #1471

Description

When building Sapling from source using make oss (or build.py --oss) on a stable Rust toolchain, the cargo build fails because vendored crates require nightly features:

error[E0554]: #![feature] may not be used on stable
  --> smallvec-1.15.2/src/lib.rs:95: specialization

as well as several crates requesting #![feature(once_cell_try)].

Context

• Internal Buck builds use a nightly toolchain where these features are permitted.
• OSS builds invoked via make oss / cargo use the system stable rustc by default.
• The Cargo.toml files are @generated by autocargo from BUCK files, so they cannot simply be hand-edited to remove the feature flags.