making rustup prepend cargo bin to path instead of append

[ducks]

A first attempt at fixing #475

I'm not sure if the change is this simple because I'm honestly not sure how to test this one.
But I changed the Toolchain::set_ldpath to prepend instead of append.

If you could share some advice on how to test I'm more than willing to do so :)

[ducks]

Looking at the CI build I would say it's not that simple

So it seems like I need to make that use a std::path::Path instead of a vec<PathBuf>

I try building this locally but don't seem to get the same issue.
(oh, some of the tests seem to be passing now. maybe that's why. yay.)

[brson]

Looks good. Thanks @rjgoldsborough !