Why run commands in test.sh in a subprocess

[tcharding]

Does anyone know why we run commands in test.sh in a subprocess?

if [ "$DO_FMT" = true ]
then
    (
        rustup component add rustfmt
        cargo fmt --all -- --check
    )
fi

We don't do this in other test.sh scripts in the stack.

(Well at least I think that is what the extra ( and ) do.)

[tcharding]

I just went ahead and did a PR to see if it gets past CI. #405