Add NVIDIA_VISIBLE_DEVICES env var in GPU monitoring

[gjulianm]

What this PR does / why we need it:

Include the NVIDIA_VISIBLE_DEVICES environment variable in the agent and system-probe containers when GPU monitoring is enabled.

This is needed in addition to the /var/run/nvidia-container-devices mount, as the latter will only be recognized by the k8s NVIDIA device plugin when accept-nvidia-visible-devices-as-volume-mount is enabled in its configuration.

Which issue this PR fixes

(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)

• fixes #

Special notes for your reviewer:

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• Chart Version bumped
• Documentation has been updated with helm-docs (run: .github/helm-docs.sh)
• CHANGELOG.md has been updated
• Variables are documented in the README.md
• For Datadog Operator chart or value changes update the test baselines (run: make update-test-baselines)

[val06]

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