test: build with emulator

[crazy-max]

relates to moby/buildkit#4949

[crazy-max]

It seems the very first run has the expected behavior but other ones for any ref seems to have some cache: https://github.com/moby/buildkit-bench/actions/runs/10958911268/job/30430282526?pr=132#step:8:50

 goos: linux
goarch: amd64
pkg: github.com/moby/buildkit-bench/test
cpu: AMD EPYC 7763 64-Core Processor                
BenchmarkBuild
BenchmarkBuild/BenchmarkBuildEmulator/ref=2024-09-17/run=1
time="2024-09-20T12:10:36Z" level=info msg="trying next host - response was http.StatusNotFound" host="localhost:40955"
    run.go:459: copied docker.io/amd64/busybox:latest@sha256:023917ec6a886d0e8e15f28fb543515a5fcd8d938edb091e8147db4efed388ee to local mirror localhost:40955/library/busybox:latest
time="2024-09-20T12:10:36Z" level=info msg="trying next host - response was http.StatusNotFound" host="localhost:40955"
    run.go:459: copied docker.io/amd64/golang:1.22-alpine@sha256:51c59ce1d82286f8c6498ad78f97528fc7896fcff59997bd02b4b76c7f4979ca to local mirror localhost:40955/library/golang:1.22-alpine
time="2024-09-20T12:10:38Z" level=info msg="trying next host - response was http.StatusNotFound" host="localhost:40955"
    run.go:459: copied docker.io/amd64/python:latest@sha256:cd07fcc5721f0d1ae2097291a30315176a997d8819e278827be0e090ba187bd2 to local mirror localhost:40955/library/python:latest
time="2024-09-20T12:10:41Z" level=info msg="trying next host - response was http.StatusNotFound" host="localhost:40955"
    run.go:459: copied docker.io/docker/dockerfile:1.9.0 to local mirror localhost:40955/docker/dockerfile:1.9.0
time="2024-09-20T12:10:43Z" level=info msg="trying next host - response was http.StatusNotFound" host="localhost:40955"
    run.go:459: copied docker.io/amd64/busybox:latest to local mirror localhost:40955/amd64/busybox:latest
time="2024-09-20T12:10:43Z" level=info msg="trying next host - response was http.StatusNotFound" host="localhost:40955"
    run.go:459: copied docker.io/arm64v8/busybox:latest to local mirror localhost:40955/arm64v8/busybox:latest
BenchmarkBuild/BenchmarkBuildEmulator/ref=2024-09-17/run=1-4         	       1	8366261553 ns/op	41067088 B/op	  268079 allocs/op
BenchmarkBuild/BenchmarkBuildEmulator/ref=2024-09-17/run=2
BenchmarkBuild/BenchmarkBuildEmulator/ref=2024-09-17/run=2-4         	       1	 410022562 ns/op	  253880 B/op	     448 allocs/op
BenchmarkBuild/BenchmarkBuildEmulator/ref=v0.15.2/run=1
BenchmarkBuild/BenchmarkBuildEmulator/ref=v0.15.2/run=1-4            	       1	 419640102 ns/op	  253904 B/op	     450 allocs/op
BenchmarkBuild/BenchmarkBuildEmulator/ref=v0.15.2/run=2
BenchmarkBuild/BenchmarkBuildEmulator/ref=v0.15.2/run=2-4            	       1	 382064265 ns/op	  253864 B/op	     448 allocs/op

@tonistiigi Not sure how to bust it for each run 🤔

Here is the logs: https://github.com/moby/buildkit-bench/actions/runs/10958911268/job/30430282526#step:10:27