Allow changing the default containerd snapshotter

### Description

Currently `CONTAINERD_SNAPSHOTTER` is set to `fuse-overlayfs` and this value cannot be overriden. Neither by adding `CONTAINERD_SNAPSHOTTER: stargz` to the `env:` section in `~/.lima/default/lima.yaml` nor by using a provision script. Please allow changing this value or set it to `stargz` whenever containerd is running in rootful mode.