Bump `CUDA_VER` to 11.5

[charlesbluca]

Addresses https://github.com/rapidsai/dask-build-environment/pull/23#discussion_r769033743; also cc @quasiben I don't think CEC should come up in this case, as this should also bump the underlying CUDA version of the base image being used:

https://github.com/rapidsai/dask-build-environment/blob/1b0ceeb3bd4b28dc9273ec71d320def17de7c068/dask.Dockerfile#L4

Will be followed up with PRs to the affected Dask repos:

• https://github.com/dask/dask/pull/8489
• https://github.com/dask/distributed/pull/5604
• https://github.com/dask-contrib/dask-sql/pull/348

[quasiben]

Thanks @charlesbluca