test: move gha cache to its own suite #5845

Job	Run time
sandbox-build (linux/amd64)	5m 10s
build (windows/amd64)	4m 6s
sandbox-build (linux/arm64)	5m 10s
build (freebsd/amd64)	3m 46s
test-freebsd-amd64	1m 6s
test-windows-amd64 (containerd, ./client)	1m 8s
test-windows-amd64 (containerd, ./cmd/buildctl)	1m 22s
test-windows-amd64 (containerd, ./worker/containerd)	1m 20s
test-windows-amd64 (containerd, ./solver)	1m 21s
test-windows-amd64 (containerd, ./frontend)	1m 18s
test-windows-amd64 (containerd, ./frontend/dockerfile)	1m 21s
test-windows-amd64 (./..., 1)	1m 20s
	28m 28s

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