Allocations incurred from `copyto!` with `FastBroadcast`

MWE:

```julia
using FastBroadcast
using Test
import ClimaCore
include(
joinpath(pkgdir(ClimaCore), "test", "TestUtilities", "TestUtilities.jl"),
)
import .TestUtilities as TU

function fast_broadcast_single_field!(Y1, dt, Y2)
x = Y1.x
@.. x += 2.0
nothing
end
# Removing dt from argument fixes allocations
function fast_broadcast_copyto!(Y1, dt, Y2)
@.. Y1 = Y2
nothing
end
@testset "Allocations in @.. broadcasting" begin
FT = Float32
for space in TU.all_spaces(FT)
Y1 = fill((; x = FT(2.0), y = FT(2.0), z = FT(2.0)), space)
Y2 = fill((; x = FT(2.0), y = FT(2.0), z = FT(2.0)), space)
Y3 = fill((; x = FT(2.0), y = FT(2.0), z = FT(2.0)), space)
dt = FT(2.0)
fast_broadcast_single_field!(Y1, dt, Y2)
p = @allocated fast_broadcast_single_field!(Y1, dt, Y2)
@test p == 0
fast_broadcast_copyto!(Y1, dt, Y2)
p = @allocated fast_broadcast_copyto!(Y1, dt, Y2)
@test_broken p == 0
end
end
```

Start by running the provided Julia MWE and confirm the allocation difference between the two @.. functions. Inspect the FastBroadcast copyto! path and compare it with the single-field broadcast case. Done means the copyto! case no longer allocates and the broken allocation assertion can pass.

Written by the indexing model from the issue text.