failure at handling globals on julia 1.12

[CarloLucibello]

This test used to pass on julia < 1.12:

using Zygote, Test


julia> global_r = 1
1

julia> function pow_global(x, n)
           global global_r
           while n > 0
             global_r *= x
             n -= 1
           end
           return global_r
         end
pow_global (generic function with 1 method)

julia> @test gradient(pow_global, 2, 3) == (12, nothing)

Now the returned gradient is (nothing, nothing) instead.