Fix `utils::copy_file` for symlink.

[reitermarkus]

When rustup-init is a symlink, create a symlink to that symlink instead of resolving it first. This is also in line with using the no-self-update feature, to allow managing the rustup version with Homebrew.

Fixes #1512.

[reitermarkus]

Ping.

[reitermarkus]

Ping.

[reitermarkus]

Ping.

[reitermarkus]

@alexcrichton, anything I can do to move this along?

[alexcrichton]

@reitermarkus unfortunately rustup doesn't have a lot of active development and I don't personally have enough time to take a deep dive here to see if this is the correct fix or not (or what sort of unintended fallout this might have)

[reitermarkus]

@alexcrichton, no worries. For what it's worth, this patch has been integrated into the Homebrew package for a month now without any bug reports, if that is any indication on correctness.

[nrc]

Given this fixes an issue that is affecting several people and seems to be working for Homebrew, I'm going to merge this. Thanks for the PR @reitermarkus !