Behavior change: Don't change TMPDIR in WithRegistriesConf

Previously, using WithRegistriesConf was setting
BigFilesTemporaryDir using buildah/internal/tmpdir, which (turns relative
$TMPDIR into absolute and) silently ignores invalid values in containers.conf
and uses /var/tmp in that case.

Without WithRegistriesConf, we would first initialize libimage with
BigFilesTemporaryDir = nil (causing libimage to determine BigFilesTemporaryDir
using a _different_ code path which fails on invalid containers.conf),
and only later set r.imageContext.BigFilesTemporaryDir.

It doesn't make sense that --registries-conf should affect TMPDIR processing
that way, and presumably the code path without --registries-conf is much
more frequently used and the one everyone expects; so drop this aspect of
WithRegistriesConf.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>