Turn on S3 Express canary via env variable

This commit sets `ENABLE_S3_EXPRESS_CANARY` to true so that S3 Express
canary starts running in `aws-sdk-rust` repo. Note that we run canary
against three releases; latest, previous, and the one before last.
This env variable is temporary, and we were originally going to turn it
on after S3 Express implementation is available in those releases
(which should be after 2024-03-15), otherwise s3_express_canary would fail.
However, since the canary already fails due to `canary-wasm` not being able
to find `aws-smithy-wasm` in `versions.toml`, so setting the said env
variable right now will not make things worse.

`canary-wasm` should start passing when run against 2024-03-15, 2024-03-14,
and 2024-03-13, and so should s3_express_canary. At that point, we can
remove `ENABLE_S3_EXPRESS_CANARY` and run s3_express_canary by default.

.github/workflows/canary.yaml

@@ -49,6 +49,7 @@ jobs:
       LAMBDA_EXECUTION_ROLE_ARN: ${{ secrets.CANARY_LAMBDA_EXECUTION_ROLE_ARN }}
       LAMBDA_TEST_S3_MRAP_BUCKET_ARN: ${{ secrets.CANARY_LAMBDA_TEST_S3_MRAP_BUCKET_ARN }}
       LAMBDA_TEST_S3_EXPRESS_BUCKET_NAME: ${{ secrets.CANARY_LAMBDA_TEST_S3_EXPRESS_BUCKET_NAME }}
+      ENABLE_S3_EXPRESS_CANARY: true
     permissions:
       id-token: write
       contents: read