Helm chart: add volumeMounts for nvidiaDriverRoot + custom env. for NFD pod

[plevart]

This is a proposed fix for #1677

With this patch, I can specify something like the following in the Helm chart custom values.yaml:

deviceListStrategy: cdi-cri
nvidiaDriverRoot: "/"
gfd:
  enabled: true
  env:
    # when deviceListStrategy=cdi-cri, there is no runc wrapper to inject the library
    - name: LD_PRELOAD
      value: /driver-root/usr/lib64/libnvidia-ml.so.1

...and the gfd Pod is happy.

I also cleaned-up and aligned the logic of this part of template with device plugin template.

[copy-pr-bot]

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