Add global qualifier to inserted globals

[serenity4]

Disambiguates global assignments that used to be allowed but that no longer are after JuliaLang/julia#57253.

It makes this error go away on nightly:

julia> using Infiltrator

julia> @infiltrate
Infiltrating top-level frame

ERROR: invalid assignment to constant anonymous.@__MODULE__. This redefinition may be permitted using the `const` keyword.
Stacktrace:
 [1] top-level scope
   @ none:1
 [2] eval(m::Module, e::Any)
   @ Core ./boot.jl:488
 [3] init_transient_eval_module(mod::Module, locals::Dict{Symbol, Any})
   @ Infiltrator ~/julia/wip3/.julia/dev/Infiltrator/src/Infiltrator.jl:533

[serenity4]

Seems like tests were failing for 1.11 before this PR: https://github.com/JuliaDebug/Infiltrator.jl/actions/runs/12650638097/job/35249552745

but the failure cancels tests for all other versions. Feel free to merge as is (if you are confident with the change) or to proceed with selectively running/fixing tests as you prefer.

[pfitzseb]

Thanks. I'll merge this as-is and figure out the 1.11 test failure later...