feat: use dd_runtime tag

[alexgallotta]

check the DD_RUNTIME env var and use it as runtime

No fallback to proc logic or default value is used to avoid the worst case scenario:

• Datadog integration picks up the AWS runtime, and set it
• lambda function with no DD_RUNTIME set will scan proc. It's still not populated, so the provided.al2023 will be read
• DD_RUNTIME is then set with a different value from the one from AWS
• each datapoint will have either AWS runtime+provided.al2023 or AWS runtime+DD_RUNTIME

in the best case scenario, DD_RUNTIME is set with the same value of the AWS runtime, or the AWS integration is disabled.

In both case, it seems that the proc scan fallback will add a value not useful and confusing, so it might be better to remove it altogether