FTBFS due to failed assertion in test_nested_env_delimiter_complex_required

Failed build log here: https://launchpadlibrarian.net/609965365/buildlog_ubuntu-kinetic-amd64.pydantic_1.9.0-1_BUILDING.txt.gz

This fails because of the `V=1` environment variable set by LP builds [1]:

```
User Environment
----------------

APT_CONFIG=/var/lib/sbuild/apt.conf
DEB_BUILD_OPTIONS=parallel=4
HOME=/sbuild-nonexistent
LANG=C.UTF-8
LC_ALL=C.UTF-8
LOGNAME=buildd
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games
SCHROOT_ALIAS_NAME=build-PACKAGEBUILD-23574084
SCHROOT_CHROOT_NAME=build-PACKAGEBUILD-23574084
SCHROOT_COMMAND=env
SCHROOT_GID=2501
SCHROOT_GROUP=buildd
SCHROOT_SESSION_ID=build-PACKAGEBUILD-23574084
SCHROOT_UID=2001
SCHROOT_USER=buildd
SHELL=/bin/sh
TERM=unknown
USER=buildd
V=1
```

The failing test case expects that `V` is unset in the execution environment, so that the default value is used.

[1] LP: #516208