perf: Use 1 cpu for performance tests

Testing shows that 1 cpus is 5%-18% faster. Using less cpus is also
better for getting more consistent results on smaller machines with
fewer performance cores.

Test results on M3 machine:

| vm     | cpus | host to vm   | vm to vm     |
|--------|------|--------------|--------------|
| vz     |    2 |  3.3 Gbits/s |  1.8 Gbits/s |
| vz     |    1 |  3.9 Gbits/s |  1.9 Gbits/s |
| qemu   |    2 |  3.5 Gbits/s |  2.2 Gbits/s |
| qemu   |    1 |  3.8 Gbits/s |  2.5 Gbits/s |

Signed-off-by: Nir Soffer <nirsof@gmail.com>

Author: nirs

test/lima.yaml

@@ -3,7 +3,7 @@ images:
   arch: "x86_64"
 - location: "https://cloud-images.ubuntu.com/releases/24.10/release/ubuntu-24.10-server-cloudimg-arm64.img"
   arch: "aarch64"
-cpus: 2
+cpus: 1
 memory: "2g"
 plain: true
 networks: