fix: cryptic error on some sysimage builds on Julia 1.8.5

[pankgeorg]

This type assert, while perfectly legal, is breaking some sysimage builds with an error that starts with

signal (11): Segmentation fault
in expression starting at REPL[3]:1
ijl_subtype_env at /cache/build/default-amdci4-2/julialang/julia-release-1-dot-8/src/subtype.c:1854
#387 at /home/jrun/.julia/packages/Pluto/YRKRh/src/webserver/Dynamic.jl:189

This is possibly related to JuliaLang/julia#44527, but in any case doesn't appear in higher versions of Julia (tested with 1.9.1+)

[github-actions]

Try this Pull Request!

Open Julia and type:

julia> import Pkg
julia> Pkg.activate(temp=true)
julia> Pkg.add(url="https://github.com/fonsp/Pluto.jl", rev="pg/morelesnaneafto")
julia> using Pluto