Add `RUSTFLAGS = aws_sdk_unstable` as environment variable when running Cargo from kotlin (#2614)

## Motivation and Context
This PR set `aws_sdk_unstable` to RUSTFALGS when running cargo from
kotlin.

It is required to enable test gated features introduced on RFC30.

No breaking changers are introduced.

## Description
add `RUSTFLAGS = aws_sdk_unstable` when running cargo from kotlin.


## Parent PR
This PR is listed as one of prerequisite PRs on this PR.

- https://github.com/awslabs/smithy-rs/pull/2615

## Testing
NA.

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