Add generic build manifest #89

Job	Run time
prepare	6s
build (x86_64, httpreply, qemu) / buildunikernel	25s
build (aarch64, httpreply, fc) / buildunikernel	33s
build (aarch64, httpreply, qemu) / buildunikernel	48s
build (x86_64, httpreply, fc) / buildunikernel	24s
buildgeneric (aarch64, httpreply-go) / buildapp	1m 2s
buildgeneric (x86_64, httpreply-go) / buildapp	30s
build (x86_64, httpreply, fc) / buildimage	18s
build (aarch64, httpreply, fc) / buildimage	27s
build (aarch64, httpreply, qemu) / buildimage	23s
buildgeneric (x86_64, httpreply-go) / sign	14s
build (aarch64, httpreply, fc) / sign	0s
build (aarch64, httpreply, qemu) / sign	0s
build (x86_64, httpreply, qemu) / buildimage	17s
buildgeneric (aarch64, httpreply-go) / sign	17s
build (x86_64, httpreply, fc) / sign	0s
manifestgeneric (httpreply, go) / manifest	18s
build (x86_64, httpreply, qemu) / sign	0s
manifest	0s
	6m 2s

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