Note in install docs that glab exists on conda-forge

As of 0545e8f2 the install docs https://gitlab.com/gitlab-org/cli/-/blob/main/docs/installation_options.md don't mention that glab exists on conda-forge: https://github.com/conda-forge/glab-feedstock

From conda-forge people can install globally into user space with Pixi

pixi global install glab

for the platforms

which is hugely useful.