cmake: emu: ensure that QEMU_PIPE is set when using sysbuild

Ensure that QEMU_PIPE variable is set when using sysbuild. This is required
because twister will use the "main" sysbuild application as the target for
"ninja run" when testing QEMU targets, which means that the "main" build
must be aware of any QEMU_PIPE setting passed by twister at build time.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>