Allow "Update GitHub Pages" to opt-in to cargo sparse registry (#2610)

## Motivation and Context
Addresses a CI failure within `Update GitHub Pages` due to the use of
the cargo sparse registry
([example](https://github.com/awslabs/smithy-rs/actions/runs/4746548497/jobs/8430328515)).

## Description
The PR allows the action to configure whether the cargo sparse registry
is used via the environment variable
[CARGO_UNSTABLE_SPARSE_REGISTRY](https://blog.rust-lang.org/2022/06/22/sparse-registry-testing.html#overview):

<img width="800" alt="Screenshot 2023-04-20 at 11 55 06 AM"
src="https://user-images.githubusercontent.com/15333866/233435686-d24ceb2f-facf-45c9-b6cb-490b8ea2b085.png

">

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Co-authored-by: Yuki Saito <awsaito@amazon.com>
Co-authored-by: Zelda Hessler <zhessler@amazon.com>