Serialization of ODEProblemExpr based models broke in 5.7

[andreasnoack]

The code below works in version 5.6 but not in 5.7 (and more recent versions).

using ModelingToolkit, SciMLBase, Serialization

@parameters t
@variables x(t)
D = Differential(t)

sys = ODESystem([D(x) ~ -0.5*x])
prob = eval(ModelingToolkit.ODEProblemExpr{false}(sys, nothing, nothing, SciMLBase.NullParameters()))

_fn = tempname()

open(_fn, "w") do f
    serialize(f, prob)
end

_cmd = "using ModelingToolkit, Serialization; deserialize(\"$_fn\")"

run(`$(Base.julia_cmd()) -e $(_cmd)`)

The failure is

ERROR: UndefVarError: ###f#261 not defined
Stacktrace:
  [1] deserialize_datatype(s::Serializer{IOStream}, full::Bool)
    @ Serialization ~/julia/usr/share/julia/stdlib/v1.6/Serialization/src/Serialization.jl:1278
...

which is the type of the ODE closure

julia> typeof(prob.f.f)
var"###f#261"

Notice that if I construct the problem via the ODEProblem constructor then I don't get the error and the closure type is different

julia> typeof(prob.f.f)
ModelingToolkit.var"#f#251"{RuntimeGeneratedFunctions.RuntimeGeneratedFunction{(Symbol("##arg#273"), Symbol("##arg#274"), :t), ModelingToolkit.var"#_RGF_ModTag", ModelingToolkit.var"#_RGF_ModTag", (0xcfab6c31, 0x4234d024, 0xc8d24f26, 0x00e8741b, 0xdef727f3)}, RuntimeGeneratedFunctions.RuntimeGeneratedFunction{(Symbol("##out#275"), Symbol("##arg#273"), Symbol("##arg#274"), :t), ModelingToolkit.var"#_RGF_ModTag", ModelingToolkit.var"#_RGF_ModTag", (0x1649a627, 0x3083de65, 0x3aff2680, 0x103a1319, 0xebfbfbe8)}}

[ChrisRackauckas]

This looks like it's likely an upstream issue with build_function due to #764 (upstream as this is now in Symbolics.jl)