slightly improve inference in precompilation code (#56084)
Avoids the
```
11: signature Tuple{typeof(convert), Type{String}, Any} triggered MethodInstance for Base.Precompilation.ExplicitEnv(::String) (84 children)
```
shown in
https://github.com/JuliaLang/julia/issues/56080#issuecomment-2404765120
Co-authored-by: KristofferC <kristoffer.carlsson@juliacomputing.com>
