CI 'pages' job sometimes fails with "htags: Negative exec line limit"

Sometimes the CI "pages" job fails with a weird error message from hflags. For example https://gitlab.com/qemu-project/qemu/-/jobs/5871592479 failed with:

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

Apparently this is due to a bug in hflags where if the environment is too large then it falls over. As diagnosed in this email thread: https://www.mail-archive.com/qemu-devel@nongnu.org/msg1014350.html this happens to us because Gitlab's CI puts the commit message of a commit into an environment variable, so if the commit message is very large then it crashes. We can work around this by clearing the CI_COMMIT_MESSAGE when invoking htags.