chore: specify number of CPU cores

Author: darora

qemu-arm64-nix.pkr.hcl

@@ -81,6 +81,7 @@ source "qemu" "cloudimg" {
   qemuargs = [
     ["-machine", "virt"],
     ["-cpu", "host"],
+    ["-smp", "cores=8,threads=1"],
     ["-device", "virtio-gpu-pci"],
     ["-drive", "if=pflash,format=raw,id=ovmf_code,readonly=on,file=/usr/share/AAVMF/AAVMF_CODE.fd"],
     ["-drive", "if=pflash,format=raw,id=ovmf_vars,file=AAVMF_VARS.fd"],