dockerfile: add delve to binaries-for-test #5321

Job	Run time
build (windows/amd64)	1m 55s
build (freebsd/amd64)	1m 40s
test-freebsd-amd64	2m 32s
test-windows-amd64 (containerd, ./client)	4m 41s
test-windows-amd64 (containerd, ./cmd/buildctl)	3m 49s
test-windows-amd64 (containerd, ./worker/containerd)	2m 32s
test-windows-amd64 (containerd, ./solver)	1m 44s
test-windows-amd64 (containerd, ./frontend)	2m 41s
test-windows-amd64 (containerd, ./frontend/dockerfile)	33m 39s
test-windows-amd64 (./..., 1)	12m 35s
	1h 7m 48s

Provide feedback