update merge script to use stable version of rust (#1697)

### Change summary

The [merge to
main](https://github.com/fastly/cli/actions/runs/23196725058) build is
failing due to new features being added to rust and us using old
versions, updating to use stable

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* [x] Have you followed the guidelines in our Contributing document?
* [x] Have you checked to ensure there aren't other open [Pull
Requests](https://github.com/fastly/cli/pulls) for the same
update/change?

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### New Feature Submissions:

* [x] Does your submission pass tests?

Author: anthony-gomez-fastly

.github/workflows/merge_to_main.yml

@@ -20,7 +20,7 @@ jobs:
         with:
           node-version: 18
       - name: "Install Rust"
-        uses: dtolnay/rust-toolchain@1.83.0 # to install tq via `make config`
+        uses: dtolnay/rust-toolchain@stable # to install tq via `make config`
       - name: Install Go
         uses: actions/setup-go@v6
         with: