For usage in CI (among other things) it can be rather usefull to direct
the build-snapshot temporary directory location to e.g. the jobs
workspace such that the central /tmp can't be filled up.

fddcef0a reverted an earlier version of this patch, claiming there
were issues with the older mktemp from the mktemp package. In reality
the initial patch was actually wrong, using --tmpdir= instead of
--tmpdir. The latter works correctly with both versions of mktemp

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>