%SBUILD_CHROOT_EXEC is unexpanded using autopkgtest chroot type

[Chris Halse Rogers]

This makes it rather difficult to use --pre-build-commands.

It appears to be because ChrootAutopkgtest.pm does not define a “get_internal_exec_string” subroutine.

This seems to be a regression; I'm pretty sure this worked at *some* time in the past.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: sbuild 0.79.0-1ubuntu1
ProcVersionSignature: Ubuntu 5.4.0-31.35-generic 5.4.34
Uname: Linux 5.4.0-31-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu27
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Tue May 26 15:50:33 2020
PackageArchitecture: all
SourcePackage: sbuild
UpgradeStatus: No upgrade log present (probably fresh install)