Fall back to fs::copy when hard_link fails.

Fall back to fs::copy when hard_link fails.

Some filesystems don't allow hard links.  Since Cargo's use of hard
links is an optimization, and not necessary for correctness, it can fall
back to a file copy when hard linking is not available.

This is one possible solution to #3098.

Caveat: this will try to copy if the hard link fails *for any reason*.
It's not clear that there's a more surgical way of handling this; Unix
tends to indicate the condition as "permission denied," not with a
granular "links not supported by filesystem" error.