tests: Remove some problematic environment variables
If autopkgtest is run in an unclean environment, this avoids having that environment leak into the package under test.
In particular, when autopkgtest itself (an Architecture: all package) is built, we must not allow internal variables set by debhelper to propagate into the Architecture: any packages that are built and tested by autopkgtest's test suite, because that will make them fail to build.
Resolves: https://salsa.debian.org/ci-team/autopkgtest/-/issues/7
This is a release blocker, and we have accumulated several RC bugs since the last release, so I want to get this in even though it's a bit of a hack.
I also tried a version of this that uses dpkg-buildpackage --sanitize-env (!504) but that doesn't actually solve this, because --sanitize-env only resets the umask and the locale: it does not scrub all environment variables that are not on an allowlist like debuild does, which is what we would actually want here.