docs: fix `cilium install` command in k3s guide
docs: fix `cilium install` command in k3s guide

Explicity set `KUBECONFIG` when using `cilium install`, otherwise
installation might target the wrong cluster.

Follow commit 606b5fe9f49f ("docs: KUBECONFIG for cilium-cli with k3s")
which already changed this in the quick start guide.

Signed-off-by: Tobias Klauser <059b8b880f8441509ec8a65b50b4c6ae74ebea76@cilium.io>

michi-covalent/cilium,cilium/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,michi-covalent/cilium,cilium/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium