fix(oss-build): allow nightly features on stable via RUSTC_BOOTSTRAP

[vegerot]

make oss failed on stable Rust 1.97.1 with:

error[E0554]: #![feature] may not be used on stable
--> smallvec-1.15.2/src/lib.rs:95: specialization
and 8 crates using #![feature(once_cell_try)]

Buck builds use a nightly toolchain internally and handle this,
but cargo oss builds use stable rustc by default.

Cargo.toml files are @generated by autocargo from BUCK files,
so we should not hand-edit them to remove the features.

Setting RUSTC_BOOTSTRAP=1 in build.py's cargo_env allows stable
cargo/rustc to accept these nightly features without requiring
users to install a nightly toolchain or override.

Fixes: cd eden/scm && make -j oss

[Copilot]

Pull request overview

This PR adjusts Sapling’s OSS cargo build environment so stable Rust can compile dependencies that use nightly-only #![feature] gates, avoiding requiring a nightly toolchain for make oss.

Changes:

• Set RUSTC_BOOTSTRAP=1 (if not already set) in cargo_env() to allow compiling crates that require nightly features during OSS cargo builds.

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[meta-codesync]

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