By default, $HOME is that of the build user. This is usually not a
problem in ephemeral build containers/chroots but can allow some files
to escape into the filesystem where `makepkg` is run outside of a chroot.
There can also be instances of generated files (e.g. cache, precompiled
bytecode) being placed into these locations and which are then relied
upon by other parts of the software.
As a concrete example, Perl6 and nim have a precompilation cache (though
well-behaved in how it is used).
`export HOME=/nonexistent` is already used by Debian in their build
tooling and so does not represent a divergence from established
practice. It also allows for badly-behaved build processes to be more
easily spotted and an issue filed upstream where appropriate.
Signed-off-by: Jonathon Fernyhough