.gitlab-ci.d/buildtest.yml: Work around htags bug when environment is large

.gitlab-ci.d/buildtest.yml: Work around htags bug when environment is large

Sometimes the CI "pages" job fails with a message like this from
htags:

$ htags -anT --tree-view=filetree -m qemu_init -t "Welcome to the QEMU sourcecode"
htags: Negative exec line limit = -371

This is due to a bug in hflags where if the environment is too large it
falls over:
https://lists.gnu.org/archive/html/bug-global/2024-01/msg00000.html

This happens to us because GitLab CI puts the commit message of the
commit under test into the CI_COMMIT_MESSAGE and/or CI_COMMIT_TAG_MESSAGE
environment variables, so the job will fail if the commit happens to
have a verbose commit message.

Work around the htags bug by unsetting these variables while running
htags.

Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2080
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240111125543.1573473-1-peter.maydell@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>