Add --audio-only flag #331

Job	Run time
Build (linux, amd64)	28s
Build (linux, 386)	33s
Build (freebsd, amd64)	36s
Build (freebsd, 386)	35s
Build (openbsd, amd64)	35s
Build (openbsd, 386)	37s
Build (dragonfly, amd64)	33s
Build (windows, amd64)	26s
Build (windows, 386)	27s
Build (darwin, amd64)	35s
Build (linux, arm, 5)	37s
Build (linux, arm, 6)	32s
Build (linux, arm, 7)	34s
Build (linux, arm64)	33s
Build (linux, mips, softfloat)	31s
Build (linux, mips, hardfloat)	27s
Build (linux, mipsle, softfloat)	32s
Build (linux, mipsle, hardfloat)	32s
Build (linux, mips64)	33s
Build (linux, mips64le)	29s
Build (linux, ppc64)	35s
Build (linux, ppc64le)	32s
Build (linux, s390x)	36s
Build (windows, arm)	29s
Build (android, arm64)	37s
Build (darwin, arm64)	29s
Build (freebsd, arm64)	33s
	14m 36s

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