If you're just modifying the existing texture, you don't even need to replicate the CreateBorder function. Just change the texture in PostCreateAuraIcon, immediately after the call to CreateBorder:
Code:
ns.CreateBorder(button, 12)
for _, tex in pairs(button.BorderTextures) do
tex:SetTexture([[Interface\AddOns\oUF_Phanx\media\BuffBorder]])
end