improve precompile generation script

[KristofferC]

This tries to improve on the gripe here: #37904 (comment).

Instead of executing everything in the REPL process, we only execute those parts that are used to precompile specific REPL things (like tab completion, showing error messages etc). The rest is executed in a normal process which will throw if an error is produced (which turns #37904 into a hard error instead of a silent one):

ERROR: LoadError: Cannot locate '(Julia)Artifacts.toml' file when attempting to use artifact 'name' in 'Main'
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:33
 [2] @artifact_str(__source__::LineNumberNode, __module__::Module, name::Any, platform::Any)
   @ Artifacts ~/julia/usr/share/julia/stdlib/v1.6/Artifacts/src/Artifacts.jl:590
 [3] @artifact_str(__source__::LineNumberNode, __module__::Module, name::Any)
   @ Artifacts ~/julia/usr/share/julia/stdlib/v1.6/Artifacts/src/Artifacts.jl:584
in expression starting at none:10
ERROR: LoadError: failed process: Process(`/home/kc/julia/usr/bin/julia -O0 --sysimage /home/kc/julia/usr/lib/julia/sys.ji --startup-file=no -Cnative -e '# Used by JuliaInterpreter
push!(Set{Module}(), Main)
---

Note that the offending line in #37904 is commented out because even with the change suggested in there, it fails.