fix: Silence home directory warning in Lambda Extensions (#1344)

* fix: Silence home directory warning in Lambda Extensions

This PR follows on to https://github.com/awslabs/smithy-rs/pull/893 which added
LAMBDA_TASK_ROOT check. When running as a [Lambda Extension][ext]
the environment variable for TASK_ROOT is redacted, so it is not
reliable for testing whether you are in Lambda.

Related commit: fbb4bc

[ext]: https://docs.aws.amazon.com/lambda/latest/dg/runtimes-extensions-api.html



* update changelog

* fixup: use AWS_LAMBDA_FUNCTION_NAME which is available in both environments

* Update CHANGELOG.next.toml

* Fix failing unit test

Co-authored-by: John DiSanti <jdisanti@amazon.com>