bug#38469: guix gc should keep around recent intermediate build

[Robert Vollmert]

> Have you tried passing the options '--gc-keep-derivations=yes’ and
> '--gc-keep-outputs=yes' to guix-daemon?

I had not. I added that right now, and first tests seem to indicate
that this helps. Thanks!

Would it be a bad idea to make this the default?

Alternatively, how about some kind of developer config fragment that
would modify a system configuration with settings that are a good
idea for Guix development, if not general use? That might also include
things like setting the $%!^@!*^% guile backtrace columns variable
for guix-daemon.