chore: memoize detected GPUs

[mudler]

Description

This pull request introduces a caching mechanism to the GPUs function in the pkg/xsysinfo/gpu.go file to improve performance by ensuring GPU information is only fetched once. The most important changes are as follows:

Caching Implementation:

• Introduced a sync.Once variable (gpuCacheOnce) to ensure the GPU information is fetched only once, even in concurrent scenarios.
• Added a gpuCache variable to store the cached GPU information and a gpuCacheErr variable to store any errors encountered during the fetch.
• Modified the GPUs function to use the sync.Once mechanism to populate the cache on the first call and return the cached data or error on subsequent calls.

Notes for Reviewers

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