fix(build): target-aware stack flag so windows-gnu toolchain links

[galaxy-luke]

Problem

Building rtk from source (or cargo install --git) on the x86_64-pc-windows-gnu toolchain fails during the final link:

error: linking with `x86_64-w64-mingw32-gcc` failed: exit code: 1
  = note: ld.exe: cannot find /STACK:8388608: No such file or directory
          collect2.exe: error: ld returned 1 exit status

This makes rtk impossible to install for anyone on the GNU toolchain (e.g. users with only MSYS2/MinGW and no MSVC Build Tools). See #1814.

Root cause

build.rs reserves a larger main-thread stack (correct — Clap + the command graph can exceed the default 1 MiB), but the flag is injected under #[cfg(windows)] only:

println!("cargo:rustc-link-arg=/STACK:8388608");

/STACK:N is MSVC linker syntax. On windows-gnu, the GNU linker (ld, driven by gcc) doesn't recognize it and treats it as a file path → link failure.

Fix

Branch on the target being built. Subtlety: #[cfg(target_env = ...)] inside build.rs reflects the host, not the target being compiled, so we read Cargo's TARGET env var instead:

• *-pc-windows-msvc → /STACK:8388608 (unchanged)
• *-pc-windows-gnu → -Wl,--stack,8388608 (GNU equivalent)

Verification

Built and installed on Windows 11 with stable-x86_64-pc-windows-gnu + MinGW-w64 (UCRT), where it previously failed with the exact error above. After the fix, cargo +stable-x86_64-pc-windows-gnu install --path . --locked succeeds; rtk --version / rtk gain work.

Fixes #1814

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[galaxy-luke]

您好！我已經完成 CLA 的簽署。再麻煩抽空協助 Review 以及核准 Workflow 執行，謝謝！