fix(scripts): git fetch origin provided commit in codegen

[trivikr]

Issue

Internal JS-4680

Description

Our internal CI was doing shallow clone till the last tag for smithy-typescript. When new tag was released in commit smithy-lang/smithy-typescript@e9a378b, our preview/release builds started failing as default commit was not available.

This PR calls git fetch origin for the provided commit in codegen, so that the commit is available.

Testing

Verified that codegen is successful by default (smithy-typescript is shallow cloned)

$ aws-sdk-js-v3> yarn generate-clients -g codegen/sdk-codegen/aws-models/acm.json -n
...
Cloning into '/local/home/trivikr/workspace/smithy-typescript'...
From https://github.com/awslabs/smithy-typescript
 * branch            d942a87081c0ca101f77b2336042773b26b0b9b1 -> FETCH_HEAD
Switched to a new branch 'temp-d942a87081c0ca101f77b2336042773b26b0b9b1'
...
Done in 19.83s.

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