cmd/internal/script: fix copying directory when symlink fails
The change fixes `linkOrCopy` to work on systems wihtout symlinks,
when copying directories. This was originally noticed on Windows
systems when the user did not have admin privs.
Fixes #73692
Change-Id: I8ca66d65e99433ad38e70314abfabafd43794b79
Reviewed-on: https://go-review.googlesource.com/c/go/+/672275 Reviewed-by: Michael Matloob <matloob@golang.org> Reviewed-by: Michael Matloob <matloob@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>