internal/testenv: use sync.OnceValues for hasSymlink
On some platforms (android, wasip1) this function is called many
times which probably results in some slowdown, especially for wasip1.
Wrap it into sync.OnceValues.
Change-Id: Id290ffd8d1e7ad806302f457e8fff2e3123b49a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/609418
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