Disallow `ADD --checksum=<SUM> <GITSRC> <DST>` #3943

Job	Run time
build (windows/amd64)	1m 36s
build (freebsd/amd64)	1m 29s
test-freebsd-amd64	21m 29s
test-windows-amd64 (containerd, ./client)	4m 31s
test-windows-amd64 (containerd, ./cmd/buildctl)	3m 6s
test-windows-amd64 (containerd, ./worker/containerd)	1m 41s
test-windows-amd64 (containerd, ./solver)	3m 26s
test-windows-amd64 (containerd, ./frontend)	3m 51s
test-windows-amd64 (containerd, ./frontend/dockerfile)	5m 32s
test-windows-amd64 (./..., 1)	11m 44s
	58m 25s

Provide feedback