[Reproducible-builds] Should reprotest set $HOME to an existent or nonexistent directory?

[Ceridwen]

It came up in discussion of this bug that setting $HOME to a
nonexistent directory causes a build to generate errors:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=829113

At the moment, reprotest sets $HOME to '/nonexistent/first-build'
and '/nonexistent/second-build', following the prebuilder script in
this.  (Neither prebuilder nor reprotest guarantee that those
directories don't exist.)  As Lunar said, one possible alternate fix
for that specific bug is to create two different temporary directories
and set $HOME to those directories instead.  Is this a good idea?

Ceridwen