buildkitd: remove sync.Once from isRootlessConfig #5741

Job	Run time
sandbox-build (linux/amd64)	6m 59s
build (windows/amd64)	2m 2s
sandbox-build (linux/arm64)	6m 58s
build (freebsd/amd64)	1m 44s
test-windows-amd64 (containerd, ./client)	7m 0s
test-windows-amd64 (containerd, ./cmd/buildctl)	2m 54s
test-freebsd-amd64	2m 32s
test-windows-amd64 (containerd, ./worker/containerd)	2m 1s
test-windows-amd64 (containerd, ./solver)	3m 23s
test-windows-amd64 (containerd, ./frontend)	3m 12s
test-windows-amd64 (containerd, ./frontend/dockerfile)	42m 2s
test-windows-amd64 (./..., 1)	5m 49s
	1h 26m 36s

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