cmd/go: skip TestScript/list_goroot_symlink on darwin

cmd/go: skip TestScript/list_goroot_symlink on darwin

The list_goroot_symlink test relies on fsys.Walk (and ultimately
syscall.Lstat) conforming to POSIX pathname resolution semantics.
POSIX requires that symlinks ending in a slash be fully resolved,
but it appears that lstat in current darwin kernels does not fully
resolve the last pathname component when it is a symlink to a symlink.

For #59586.
For #35678.

Change-Id: I37526f012ba94fa1796b33109a41c3226c967d3e
Reviewed-on: https://go-review.googlesource.com/c/go/+/484216
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Bypass: Bryan Mills <bcmills@google.com>