ecs-gpu-init: Fix go lint

[yeazelm]

Description of changes:
The new lint rules for Go want us to check the return code. For this defer its not super useful, but we can still emit a log entry on shutdown just in case things go poorly.

Testing done:
cargo make check-golangci-lint now passes. It used to say:

 Running [/home/runner/golangci-lint-1.59.0-linux-amd64/golangci-lint run --path-prefix=sources/ecs-gpu-init] in [/home/runner/work/bottlerocket/bottlerocket/sources/ecs-gpu-init] ...
  Error: sources/ecs-gpu-init/cmd/ecs-gpu-init/main.go:28:21: Error return value is not checked (errcheck)
  	defer nvml.Shutdown()
  	                   ^
  
  Error: issues found

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.