Bootstrap does not set `cfg(bootstrap)` for build scripts

[tgross35]

I was experimenting with some changes to compiler-builtins that were sensitive to bootstrap. --cfg=bootstrap does get passed to the crates but doesn't seem to pass to build scripts. E.g. for the build script:

Running `~/projects/rust/build/bootstrap/debug/rustc ~/projects/rust/build/bootstrap/debug/rustc --crate-name build_script_build --edition=2021 ~/.cargo/git/checkouts
/compiler-builtins-ef822f47a433bc09/4c7c9af/build.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type bin --emit=dep-info,link -C embed-bitcode=no -C codegen-units=1
0000 -C debuginfo=line-tables-only -C overflow-checks=off --cfg 'feature="compiler-builtins"' --cfg 'feature="core"' --cfg 'feature="default"' --cfg 'feature="rustc-dep-of-std"' --check-cfg 'cfg(docsrs,test)
' --check-cfg 'cfg(feature, values("c", "cc", "compiler-builtins", "core", "default", "mangled-names", "mem", "no-asm", "no-f16-f128", "public-test-deps", "rustc-dep-of-std"))' -C metadata=63a45c0074c9ebec -
C extra-filename=-23c20e83882c80e9 --out-dir ~/projects/rust/build/x86_64-unknown-linux-gnu/stage0-std/release/build/compiler_builtins-23c20e83882c80e9 -L dependency=~/projects/rust/b
uild/x86_64-unknown-linux-gnu/stage0-std/release/deps --cap-lints allow -Z binary-dep-depinfo`

But for the crate itself:

~/projects/rust/build/bootstrap/debug/rustc ~/projects/rust/build/bootstrap/debug/rustc --crate-name compiler_builtins --edition=2021 ~/.cargo/git/checkouts/compiler-builtins-ef822f47a433bc09/4c7c9af/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=207 --crate-type lib --emit=dep-info,metadata,link -C opt-level=3 -C embed-bitcode=no -C codegen-units=10000 -C debuginfo=line-tables-only -C debug-assertions=on -C overflow-checks=off --cfg 'feature="compiler-builtins"' --cfg 'feature="core"' --cfg 'feature="default"' --cfg 'feature="rustc-dep-of-std"' --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values("c", "cc", "compiler-builtins", "core", "default", "mangled-names", "mem", "no-asm", "no-f16-f128", "public-test-deps", "rustc-dep-of-std"))' -C metadata=5a83d879a97eb522 -C extra-filename=-963761dda15d3919 --out-dir ~/projects/rust/build/x86_64-unknown-linux-gnu/stage0-std/s390x-unknown-linux-gnu/release/deps --target s390x-unknown-linux-gnu -C linker=s390x-linux-gnu-gcc -L dependency=~/projects/rust/build/x86_64-unknown-linux-gnu/stage0-std/s390x-unknown-linux-gnu/release/deps -L dependency=~/projects/rust/build/x86_64-unknown-linux-gnu/stage0-std/release/deps --extern core=~/projects/rust/build/x86_64-unknown-linux-gnu/stage0-std/s390x-unknown-linux-gnu/release/deps/librustc_std_workspace_core-5c763a693a66c7d8.rmeta --cap-lints allow --cfg=bootstrap ...

Is this behavior intentional?

[bjorn3]

cc #138035

[tgross35]

I don't think this matters anymore with the bootstrap stage rework