document nsjail issue when building on Ubuntu 24.04

[inthewaves]

https://grapheneos.org/build#build-dependencies lists Ubuntu 24.04 LTS as one of the officially supported operating systems for building GrapheneOS.

However, due to https://discourse.ubuntu.com/t/ubuntu-24-04-lts-noble-numbat-release-notes/39890#p-99950-unprivileged-user-namespace-restrictions-15, building AOSP on Ubuntu 24.04 might run into issues with nsjail. It would be worth it to document a proper way to resolve this, e.g. adding an AppArmor profile.

https://www.maksonlee.com/making-aosps-nsjail-work-with-apparmor-ubuntu-24-04 explores this.

[inthewaves]

https://bugs.launchpad.net/ubuntu/+source/apparmor/+bug/2063976 has further discussion with one of the AppArmor maintainers