for armv6 do not use webrtc sysroot #230

Job	Run time
build (Debug, arm, armv6l, https://downloads.sourceforge.net/project/raspberry-pi-cross-compilers...	1s
build (Release, arm, armv7l, https://downloads.sourceforge.net/project/raspberry-pi-cross-compile...	21m 26s
build (Debug, arm, armv7l, https://downloads.sourceforge.net/project/raspberry-pi-cross-compilers...	21m 26s
build (Release, arm64, arm64, aarch64-linux-gnu)	20m 20s
build (Debug, arm64, arm64, aarch64-linux-gnu)	21m 26s
build (Release, arm, armv6l, https://downloads.sourceforge.net/project/raspberry-pi-cross-compile...	20m 4s
	1h 44m 43s

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