CI: use macos-15-intel to build the x86_64 binary

.github/workflows/ci.yml

@@ -149,7 +149,7 @@ jobs:
               {"os": "ubuntu-24.04", "python-version": "3.12", "toxenv": "py312-fuse3"},
               {"os": "ubuntu-24.04", "python-version": "3.13", "toxenv": "py313-fuse3"},
               {"os": "ubuntu-24.04", "python-version": "3.14", "toxenv": "py314-fuse3"},
-              {"os": "macos-15", "python-version": "3.11", "toxenv": "py311-none", "binary": "borg-macos-15-x86_64-gh"},
+              {"os": "macos-15-intel", "python-version": "3.11", "toxenv": "py311-none", "binary": "borg-macos-15-x86_64-gh"},
               {"os": "macos-15", "python-version": "3.11", "toxenv": "py311-none", "binary": "borg-macos-15-arm64-gh"}
             ]
           }'