Fix registerGenerateCargoWorkspaceTask() to declare the output file. (#3954)

## 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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