.gitlab-ci.d/buildtest.yml: Unset CI_COMMIT_DESCRIPTION for htags

.gitlab-ci.d/buildtest.yml: Unset CI_COMMIT_DESCRIPTION for htags

In commit 52a21689cd829 we added a workaround for a bug in older
versions of htags where they fail with a weird error message if the
environment is too large.  However, we missed one variable which
gitlab CI can set to the body of the commit message:
CI_COMMIT_DESCRIPTION.

Add this to the variables we unset when running htags, so that
the 'pages' job doesn't fail if the most recent commit happens
to have a very large commit message.

Cc: qemu-stable@nongnu.org
Fixes: 52a21689cd8 (".gitlab-ci.d/buildtest.yml: Work around htags bug when environment is large")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20250916163030.1467893-1-peter.maydell@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>