Go: Fix shadowed variable in RunListWithEnv

[mbg]

This PR does two things and there is a commit for each item:

• Some Go toolchain related functions from util.go are moved to toolchain.go which is a better fit for them. This commit does not change any behaviour.
• In the process of doing this, I noticed that there was a variable being shadowed in RunListWithEnv and fixed this

[owen-mc]

I think the shadowing was probably deliberate, since it's the same value but with a different type, but I have no problem with changing it.

[mbg]

@owen-mc to be clear, the shadowing caused an issue in the else branch that got flagged by the Go compiler because the err in scope there was the err that shadows the outer err. And that's a nil value in the else branch. By renaming the inner err, the outer err is no longer shadowed in the else branch and therefore, if we hit that branch, the error message is printed there as expected rather than a nil value.

[owen-mc]

Ah yes, that is a problem.