Update nightly for Security audit GitHub action (#1254)

## Motivation and Context
The Security audit GitHub action has been broken for a month. This PR
updates nightly version to fix it ([the same nightly version used by
canary)](https://github.com/awslabs/aws-sdk-rust/blob/main/.github/workflows/canary.yaml#L11).

## Testing
Confirmed that Security audit
[passed](https://github.com/awslabs/aws-sdk-rust/actions/runs/13338282531)
against this feature branch.

----

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Author: ysaito1001

.github/workflows/audit.yaml

@@ -4,7 +4,7 @@ on:
     - cron: "0 0 * * *"
   workflow_dispatch:
 env:
-  rust_version: nightly-2023-07-31
+  rust_version: nightly-2024-09-04
 
   # RUSTSEC-2020-0071 , RUSTSEC-2020-0159
   # chrono, a Rust date-time crate we use for timestamp parsing was added