fix llvmdev_build.yml to resolve toolchain incompatibility on osx-64

[swap357]

ld: unsupported tapi file type '!tapi-tbd' in YAML file '/Applications/Xcode_15.2.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib/libSystem.tbd' for architecture x86_64
    clang-14: error: linker command failed with exit code 1 (use -v to see invocation)
    ninja: build stopped: subcommand failed.

The above error is mostly caused when using conda build under conda env on (intel-based) osx-64 systems.
It's because the newer toolchains are incompatible with conda build.
To make this work, we download older toolchain and have conda build point to it using CONDA_BUILD_SYSROOT var.

--variants '{"CONDA_BUILD_SYSROOT": "/opt/MacOSX10.10.sdk"}'

Reference:

• conda build doc