Bump nightly version to nightly-2025-08-06 (#1340)

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## Motivation and Context
<!-- Why is this change required? What problem does it solve? -->
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We have been seeing some failures in the Security Audit action
([ex](https://github.com/awslabs/aws-sdk-rust/actions/runs/16791581976))
due to our version of `nightly` being too old. I bumped the version to
`nightly-2025-08-06` for both the Security Audit and Canary actions.

Note: I am concurrently making this change to smithy-rs to align all of
our nightly versions: smithy-lang/smithy-rs#4254

## Testing
Successfully an both actions against this branch:
Security Audit:
https://github.com/awslabs/aws-sdk-rust/actions/runs/16815630530
Canary: https://github.com/awslabs/aws-sdk-rust/actions/runs/16815660696

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choice.

Author: landonxjames

.github/workflows/audit.yaml

@@ -4,7 +4,7 @@ on:
     - cron: "0 0 * * *"
   workflow_dispatch:
 env:
-  rust_version: nightly-2024-09-04
+  rust_version: nightly-2025-08-06
 
   # RUSTSEC-2020-0071 , RUSTSEC-2020-0159
   # chrono, a Rust date-time crate we use for timestamp parsing was added

.github/workflows/canary.yaml

@@ -8,7 +8,7 @@ on:
       - "canary*"
 env:
   tool_rust_version: 1.86.0
-  rust_nightly_version: nightly-2025-05-04
+  rust_nightly_version: nightly-2025-08-06
 
 jobs:
   generate-canary-matrix: