solver: use toSelectors to filter root paths instead of custom logic #2225

Job	Run time
prepare	8s
binaries (darwin/amd64)	59s
binaries (darwin/arm64)	1m 1s
binaries (linux/amd64)	2m 5s
binaries (linux/arm/v7)	2m 2s
binaries (linux/arm64)	2m 0s
binaries (linux/s390x)	2m 5s
binaries (linux/ppc64le)	2m 3s
binaries (linux/riscv64)	2m 16s
binaries (windows/amd64)	1m 4s
binaries (windows/arm64)	1m 13s
test / prepare	2m 10s
test / run (containerd, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	6m 48s
test / run (containerd-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integr...	8m 58s
test / run (containerd-1.6, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	7m 17s
test / run (containerd-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./fronte...	8m 10s
test / run (oci, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	5m 28s
test / run (oci-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	5m 31s
test / run (oci-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, int...	5m 4s
test / run (./..., 1, integration gateway)	3m 56s
test / run (./client, containerd, nydus, integration)	6m 33s
test / run (./client, oci, nydus, integration)	5m 13s
test / run (./..., nydus, 1, integration)	3m 48s
image	7m 22s
image (rootless)	7m 30s
release	10s
	1h 40m 54s

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