`std::format` (C++20 `<format>`) pulls in `std::to_chars` for
floating-point formatting, which is only available on macOS 13.3+. The
Mac x64 CI job was targeting macOS 13.0, causing build errors in
`src/ast/`.
## Changes
Updated `nightly.yml` to raise the macOS deployment target for the x64
build:
- **`MACOSX_DEPLOYMENT_TARGET`**: `"13.0"` → `"13.3"` in the
`mac-build-x64` job
- **`--os` flag**: `osx-13.0` → `osx-13.3` in the `mk_unix_dist.py`
invocation
This matches the existing `mac-build-arm64` job, which already targets
macOS 13.3, and allows `std::format` to be used freely in `src/ast/`
without workarounds.
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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
