Fix registerGenerateCargoWorkspaceTask() to declare the output file.

[deven]

Motivation and Context

When I tried to build the main branch with ./gradlew assemble, I got the following error. This commit fixes the problem.

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':aws:sdk-adhoc-test:generateCargoWorkspace'.
> java.io.FileNotFoundException: /home/deven/git/smithy-rs/aws/sdk-adhoc-test/build/smithyprojections/sdk-adhoc-test/Cargo.toml (No such file or directory)

Description

Fixes registerGenerateCargoWorkspaceTask() to declare the output file correctly when creating the Cargo.toml file, so Gradle can track the dependencies properly.

Testing

Without this change, ./gradlew clean; ./gradlew assemble failed every time. With this change, it works.

Checklist

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[landonxjames]

Canary: https://github.com/smithy-lang/smithy-rs/actions/runs/12529492917