fix: Build Ghostty for baseline CPUs (#13352)
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### Description
Build vendored libghostty-vt with Zig's baseline CPU target so
distributed `turbo` binaries do not inherit CPU features from release
runners. This prevents `STATUS_ILLEGAL_INSTRUCTION` crashes on older
x86_64 Windows machines.
Fixes #13346.
### Testing Instructions
- `cargo fmt --all -- --check`
- `cargo build --release -p libghostty-vt-sys` with Zig 0.15.2
- `cargo test -p turborepo-ghostty`
- `cargo test -p turbo --test command_test
test_version_flag_matches_version_txt -- --exact`
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Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: Anthony Shew <anthonyshew@users.noreply.github.com>