build: force gnu ABI for vendored zig build under Nix sandbox
## Problem
`nix flake check` (what NixCI runs) fails compiling the vendored Zig `simdutf` library:
```
'bits/alltypes.h' file not found
... Encountered error: FileNotFound, falling back to default ABI and dynamic linker.
```
`crates/libghostty-vt-sys/build.rs` only passes `-Dtarget` to `zig build` when cross-compiling; for native builds it lets Zig auto-detect the host. That detection probes the host dynamic linker, which the **Nix sandbox hides**, so Zig falls back to a libc whose headers aren't on the include path and the C++ compile blows up. It works on a dev machine (no sandbox) but never inside a sandboxed Nix builder.
## Fix
- **`build.rs`**: add an optional `LIBGHOSTTY_VT_SYS_ZIG_TARGET` env override. When set, it's passed straight through as `-Dtarget`, bypassing native detection. Outside Nix the variable is unset and native auto-detection is unchanged.
- **`flake.nix`**: set `LIBGHOSTTY_VT_SYS_ZIG_TARGET = "native-native-gnu"` for Linux builds — keeps native arch/os/cpu while forcing the gnu ABI so the build doesn't depend on the detection the sandbox breaks. Darwin is untouched.
## Testing
`nix flake check -L` passes in the sandbox (`all checks passed!`), including the doctests.