[rustbuild] --build flag should have priority over value specified in config.toml

[nagisa]

python x.py test --host=x86_64-unknown-linux-gnu --stage=2 --target=x86_64-unknown-linux-gnu --build=x86_64-unknown-linux-gnu
    Finished debug [unoptimized] target(s) in 0.0 secs
...
Building stage0 std artifacts (i686-unknown-linux-gnu -> i686-unknown-linux-gnu)

where all build was uncommented and set to i686-unknown-linux-gnu in config.toml. It seems weird that it is still building i686 when all possible flags specify x86_64. Works as expected if build is commented out as far as I can tell.