fix: update nightly.yml mac-build-x64 to macOS 13.3 for C++20 compatibility (#10082)

`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.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

.github/workflows/nightly.yml

@@ -34,7 +34,7 @@ jobs:
     runs-on: macos-latest
     timeout-minutes: 90
     env:
-      MACOSX_DEPLOYMENT_TARGET: "13.0"
+      MACOSX_DEPLOYMENT_TARGET: "13.3"
     steps:
       - name: Checkout code
         uses: actions/checkout@v7.0.0
@@ -45,7 +45,7 @@ jobs:
           python-version: '3.x'
       
       - name: Build
-        run: python scripts/mk_unix_dist.py --dotnet-key=$GITHUB_WORKSPACE/resources/z3.snk --arch=x64 --os=osx-13.0
+        run: python scripts/mk_unix_dist.py --dotnet-key=$GITHUB_WORKSPACE/resources/z3.snk --arch=x64 --os=osx-13.3
             
       - name: Validate libz3.dylib and z3 architecture (must be x86_64)
         run: |