sadt: Set AUTOPKGTEST_TMP environment variable

Dmitry Shachnevrequested to merge
mitya57/devscripts:autopkgtest-tmp into master

Autopkgtest sets it since version 4.0 (June 2016).

Inspired by @onovy’s mass commits in python-team which change $ADTTMP to $AUTOPKGTEST_TMP in tests.

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