fix(build): use the GNU linker's stack flag on *-pc-windows-gnu

[kamusmg]

Summary

• rtk does not build at all on x86_64-pc-windows-gnu:

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

• build.rs emits cargo:rustc-link-arg=/STACK:8388608, which is MSVC link.exe syntax. GNU ld reads /STACK:8388608 as a file path and aborts. CI only covers windows-latest (msvc), so the gnu target has been silently unbuildable.

• Emit -Wl,--stack,8388608 there instead, keeping /STACK: for msvc so the original intent (a larger main-thread stack for clap's command graph) is preserved on both.

• Also switches the gate from #[cfg(windows)] to CARGO_CFG_TARGET_OS/CARGO_CFG_TARGET_ENV. Inside a build script #[cfg(windows)] describes the host compiling build.rs, not the target, so today it fires when cross-compiling away from Windows and misses cross-compiling toward it.

Test plan

• cargo fmt --all -- --check — clean
• cargo clippy --all-targets — clean
• cargo test --all on x86_64-pc-windows-gnu — 2511 passed, 0 failed (this target could not produce a binary before this change)
• Manual: rtk --version, rtk grep, rtk find, rtk git status all start and run — the larger stack is still applied, just spelled for the right linker

[CLAassistant]

All committers have signed the CLA.

[bharvey42]

Confirming this from the user side, and it's worse than a nicety: current develop (aa40853) does not link at all on x86_64-pc-windows-gnu. build.rs emits the MSVC-only /STACK:8388608 under #[cfg(windows)]; GNU ld reads it as a file path and aborts. CI only covers windows-latest (msvc), so the target has been silently unbuildable on mainline.

This patch fixes it. I've been carrying it in a local fork build since 2026-08-08 and building rtk on that target daily since, with no issues. It still merges cleanly with develop as of today.

Worth noting for triage priority: this also blocks validating any other Windows PR from a gnu toolchain, since you can't build the baseline to compare against. I hit exactly that today and had to apply this patch locally just to run cargo test on develop.