bug: chisel 1.3.0 --arch requirement

[lczyk]

in chisel 1.3.0 we got v3-essentials. this allows us to do arch-specific essentials, but for that to work we need to pass the --arch flag: canonical/chisel#256

should we add the --arch flag to install_slices helper such that these kind of slices work?

this could be a short-term fix, but in general all the slices should be happily installable without the --arch flag, so long-tern we need to have a better plan. this depends on the release strategy around canonical/chisel#256 aka will it land in 1.4.0 or 1.3.1; as well as whether we support non-latest patch versions of chisel or not.