remove unnecessary build arg in ci-wheel Dockerfile

[jameslamb]

Contributes to rapidsai/build-planning#7.

Removes one unnecessary build argument in the ci-wheel Dockerfile.

Since build arg REAL_ARCH isn't used in this FROM, specifying it before that has no effect:

ci-imgs/ci-wheel.Dockerfile

Lines 3 to 5 in a02c2e8

	ARG REAL_ARCH=x86_64
	ARG BASE_IMAGE=nvcr.io/nvidia/cuda:${CUDA_VER}-devel-${LINUX_VER}

Per https://docs.docker.com/engine/reference/builder/#scope

An ARG instruction goes out of scope at the end of the build stage where it was defined.

[vyasr]

LGTM. Seems like the CPU_ARCH is used once (for downloading sccache)

[vyasr]

Also related question, is it possible to infer CPU_ARCH from REAL_ARCH or vice versa? I always forget the fine points of the differences between things like x86_64 and amd64.

[ajschmidt8]

Also related question, is it possible to infer CPU_ARCH from REAL_ARCH or vice versa? I always forget the fine points of the differences between things like x86_64 and amd64.

There's likely some additional refactoring that can be done here, but we can save that for the future. Divye and I had a chat about this a while ago too.

For now, we'll merge this change as it is.

Thanks, James!