fix(envbox): work with cgroupv2

[johnstcn]

Fixes https://github.com/coder/envbox/issues/28

• No longer fails when trying to copy outer container quota to inner container. Warns in buildlog instead.
• Adds logic to determine current cgroup from /proc/self/cgroup
• Adds logic to read cpu.max from /sys/fs/cgroup//cpu.max
• Adds logic to write cpu.max to /sys/fs/cgroup/docker//init.scope/cpu.max
• Adds more unit tests around all of the above for both cgroupv1 and cgroupv2

Tested on:

• Rocky 9.1, kernel 5.14.0-162.23.1.el9_1.x86_64
• k3s v1.26.4
• Coder v0.23.2

Tested with both systemd.unified_cgroup_hierarchy=1 and systemd.unified_cgroup_hierarchy=0.

You can test yourself with the envbox image gcr.io/coder-dev-1/coder-cian/envboxv2:pr34