downgrade go-difflib and go-spew to tagged releases #5843

Job	Run time
build (windows/amd64)	13m 26s
build (freebsd/amd64)	13m 41s
sandbox-build (linux/amd64)	6m 46s
sandbox-build (linux/arm64)	6m 42s
test-freebsd-amd64	2m 27s
test-windows-amd64 (containerd, ./client)	7m 45s
test-windows-amd64 (containerd, ./cmd/buildctl)	2m 46s
test-windows-amd64 (containerd, ./worker/containerd)	2m 59s
test-windows-amd64 (containerd, ./solver)	2m 45s
test-windows-amd64 (containerd, ./frontend)	2m 50s
test-windows-amd64 (containerd, ./frontend/dockerfile)	39m 1s
test-windows-amd64 (./..., 1)	5m 6s
	1h 46m 14s

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