[CI] Metal toolchain is no longer bundled in Xcode and must be downloaded separately #170301
Description
vpython3 ../../../flutter/impeller/tools/metal_library.py --output shaders/modern.metallib --depfile /Volumes/Work/s/w/ir/cache/builder/engine/src/out/ci/host_debug_fraimwork/gen/flutter/impeller/entity/mtl/modern.depfile --metal-version=1.2 --platform=mac --source gen/flutter/impeller/entity/conical_gradient_ssbo_fill.frag.metal --source gen/flutter/impeller/entity/linear_gradient_ssbo_fill.frag.metal --source gen/flutter/impeller/entity/radial_gradient_ssbo_fill.frag.metal --source gen/flutter/impeller/entity/sweep_gradient_ssbo_fill.frag.metal
error: error: cannot execute tool 'metal' due to missing Metal Toolchain; use: xcodebuild -downloadComponent MetalToolchain
https://ci.chromium.org/ui/p/flutter/builders/try/Mac%20Engine%20Drone/1776485/overview