Simplify loading code from stdin during precompilation.

[maleadt]

Implements #43191 (comment)

[fredrikekre]

I believe this works without the -, and would have worked even before #43191 too, e.g.:

julia> open(`julia --startup-file=no`, "w", stdout) do proc
           write(proc, "println(\"hello, world\")")
       end;
hello, world

[vtjnash]

This selects file-mode code execution. While leaving out the - would select repl-mode code execution. Previously, I think this used Serialize, so it was slowly simplified to this form, and left being a bit more verbose than necessary.