use sha instead of ref for target_commitish #99

Job	Run time
build-docker-normal / build-docker-image (arm/v7)	27s
build-docker-normal / build-docker-image (s390x)	25s
build-docker-normal / build-docker-image (arm64)	26s
build-docker-normal / build-docker-image (amd64)	35s
build-docker-normal / build-docker-image (riscv64)	33s
build-docker-normal / build-docker-image (ppc64le)	34s
build-docker-normal / merge	0s
build-docker-normal / cleanup-docker	0s
cleanup-docker-cache	-1s
build-docker-minimal	-1s
	2m 58s

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