Work around a bug on aarch64[1] where `qemu-img` would hang
about a third of the time when converting images. To be able
to activate the work-around based on the environment, i.e.
only on certain distributions, introduce an environment
variable, `OSBUILD_QEMU_IMG_COROUTINES`, that is set in the
runner and then picked up in the assembler.
[1] https://bugs.launchpad.net/qemu/+bug/1805256