enforce not running with startup file when selecting dynamic artifacts and improve error message

[KristofferC]

Even if a user runs with --startup-file=1, it can cause problems to run this with the startup file enabled since the process collects output from stdout. This causes for example JuliaLang/julia#45583.

Also, in case the script outputs invalid TOML, it was hard to know what package caused this. Now it would write the error as:

ERROR: failed to parse TOML output from running "/home/kc//Pkg.jl/DynamicArtifact/.pkg/select_artifacts.jl", got: 
TOML Parser error:
none:1:6 error: expected equal sign after key
  Setup successful
       ^