Add -C option for CPU quota limit on build containers

[AKhozya]

Summary

This PR adds a -C option to set a CPU quota on systemd-nspawn build containers, similar to the existing -M option for memory limits.

Motivation

When running rebuilderd on a shared server, it's useful to limit CPU usage to prevent resource contention with other services. The existing -M option provides memory limits, but there's no equivalent for CPU.

Changes

• Added MAX_CPU environment variable support
• Added -C <value> command-line option (e.g., -C 500% for 5 cores)
• CPU quota is passed to systemd-nspawn via --property="CPUQuota=..."

Usage

# Command line
repro -C 500% package.pkg.tar.zst         # Limit to 5 CPU cores
repro -C 1200% -M 24G package.pkg.tar.zst # 12 cores, 24GB RAM

# Environment variable (useful for systemd services)
Environment=MAX_CPU=500%

Testing

Tested on Arch Linux with rebuilderd:

• Verified nspawn containers receive the --property=CPUQuota=... argument
• Verified cgroup cpu.max is correctly set (e.g., 500000 100000 for 500%)
• Builds complete successfully with CPU limits applied

$ ps aux | grep nspawn
systemd-nspawn -q --as-pid2 --register=no --pipe --property=CPUQuota=500% ...

$ cat /sys/fs/cgroup/.../cpu.max
500000 100000

[AKhozya]

@jelly @Foxboron please review, this one helps me to avoid all CPU consumed in my homelab so k8s resouces don't degrade in peaks :)

[Foxboron]

LGTM, but generally I suspect we should figure out if naming the systemd-nspawn session and/or advicing people to run this under systemd-run is better then trying to include all the different systemd options into repro.sh.

[AKhozya]

Thanks! Yep, that makes sense for the future. If no other comments - would be great to have it merged and potential new release :)

[AKhozya]

@Foxboron @kpcyrd @jelly may I ask you to merge it? Hate to use forks :)

[kpcyrd]

This change has been published as archlinux-repro-20260119-1-any.pkg.tar.zst, thanks for your contribution!