Fix implicit `convert(String, ...)` in several places (#56174)
This removes several `convert(String, ...)` from this code, which really
shouldn't be something we invalidate on in the first place (see
https://github.com/JuliaLang/julia/issues/56173) but this is still an
improvement in code quality so let's take it.
(cherry picked from commit a7521ea86952168c2f342b4c2275340ea7726a94)