improve type stability of process_overrides(artifact_dict::Dict, pkk_uuid::Base.UUID)

[ranocha]

This fixes some invalidations when loading Static.jl.

I used the following code to find this problem:

julia> import Pkg; Pkg.activate(temp=true); Pkg.add("Static")

julia> using SnoopCompileCore; invalidations = @snoopr(using Static); using SnoopCompile

julia> trees = invalidation_trees(invalidations);

julia> import Artifacts; ftrees = filtermod(Artifacts, trees)
1-element Vector{SnoopCompile.MethodInvalidations}:
 inserting !(::False) in Static at ~/.julia/packages/Static/sVI3g/src/Static.jl:427 invalidated:
   mt_backedges: 1: signature Tuple{typeof(!), Any} triggered MethodInstance for Artifacts.process_overrides(::Dict{String, Any}, ::Base.UUID) (3 children)

This PR gets us one step closer to finally fixing SciML/Static.jl#77.

I suggest the labels latency and backport-1.8.

[ranocha]

Can we get the backport-1.8 label here, please? It would be great to fix SciML/Static.jl#77 in Julia v1.8.2.