`--sysroot` from rustflags is ignored for proc-macro runtime lib search paths

## Problem `cargo careful test` fails to run unit tests for proc-macro crates since rust-lang/rust@887804d5686 The test executable is compiled successfully, but the dynamic loader cannot find the `libstd` built by cargo-careful: ```console Running unittests src/lib.rs (target/debug/build/pm-repro/e6767d554e5978c6/out/pm_repro-e6767d554e5978c6) .../pm_repro-e6767d554e5978c6: error while loading shared libraries: libstd-120c9d4d8f02a466.so: cannot open shared object file: No such file or directory error: test failed, to rerun pass `--lib` ``` This is likely due to rust-lang/cargo#17276. Before that PR, Cargo probed `--print=sysroot` as part of the target probe, which passed custom rustflags to the probing invocation: ```console rustc <rustflags> [--target <target>] --print=sysroot ... ``` After, it became a plain `rustc --print=sysroot` cargo-careful seems to use <https://crates.io/crates/rustc_build_sysroot> to build a custom std and invokes Cargo with `--sysroot <careful-sysroot>`. The actual test binary is linked against the custom libstd from cargo-careful sysroot, hence the failure. ## Steps ```console cargo install cargo-careful@0.4.10 --locked rustup component add rust-src --toolchain nightly cargo new --lib pm-repro printf '\n[lib]\nproc-macro = true\n' >> pm-repro/Cargo.toml cd pm-repro && cargo +nightly careful test ``` ## Possible Solution(s) A few options in my mind: * Revert rust-lang/cargo#17276 * Respect custom rustflags, though we then need to think about target rustflags and host rustflags interacdtion, which is a bit annoying. Otherwise we couldn't cover below settings. ```toml [target.aarch64-unknown-linux-gnu] rustflags = ["--sysroot", "/opt/sysroots/aarch64"] [target.x86_64-unknown-linux-gnu] rustflags = ["--sysroot", "/opt/sysroots/x86_64"] ``` ## Notes Original downstream report: RalfJung/cargo-careful#55 ## Version ```text GOOD rustc 1.99.0-nightly (7608eb7b0 2026-08-05) Cargo 7c83d4cc0953b81d823e47d640c64da9b8bd4fac (2026-07-29) BAD rustc 1.99.0-nightly (84b36a78a 2026-08-06) Cargo c79e8f89441b3e73d6d65d125c0c745792808c74 (2026-08-04) ```