autopkgtest/sbuild environment variables: LC_ALL, HOME, XDG_RUNTIME_DIR etc

[Julian Gilbey <julian@d-and-j.net>]

When I run autopkgtest using sbuild on my own machine, I sometimes get
warnings or breakages because certain environment variables are either
unset or have "broken" values, whereas I see that on ci.debian.net and
the buildds, the builds run without problem.

So clearly there are certain environment variables that are being set
on these Debian machines that are not documented (or at least not
anywhere that I could find).

The particular environment variables that I have so far stumbled upon
are HOME (pointing to where?), LC_ALL (or something similar; what is
its setting?) and XDG_RUNTIME_DIR; there are probably others I am
unaware of.

It would be really useful to be able to set up my local sbuild
environment in the same way as the Debian machines (buildd and
ci.debian.net) for testing purposes.

If anyone can shed some light on this, I'd be very grateful!  And then
perhaps this information could be added to sbuild-setup(7).

Thanks!

   Julian