pa11y-ci: Disable AppArmor instead of using `--no-sandbox`

[jpmckinney]

https://chromium.googlesource.com/chromium/src/+/main/docs/security/apparmor-userns-restrictions.md

echo 0 | sudo tee /proc/sys/kernel/apparmor_restrict_unprivileged_userns

like at https://github.com/pa11y/pa11y-ci/blob/f4d28387b2b7a866454a4a41a7ba2e98f9a46201/.github/workflows/tests.yml#L35-L39

h/t pa11y/pa11y-ci#228 (comment)