chore(deps): update actions/checkout action to v4 (#26)

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This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [actions/checkout](https://togithub.com/actions/checkout) | action |
major | `v3` -> `v4` |

---

### Release Notes

<details>
<summary>actions/checkout (actions/checkout)</summary>

###
[`v4`](https://togithub.com/actions/checkout/blob/HEAD/CHANGELOG.md#v400)

[Compare Source](https://togithub.com/actions/checkout/compare/v3...v4)

- [Support fetching without the --progress
option](https://togithub.com/actions/checkout/pull/1067)
-   [Update to node20](https://togithub.com/actions/checkout/pull/1436)

</details>

---

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Diff for: .github/workflows/ci.yaml

@@ -14,7 +14,7 @@ jobs:
     name: Find exercises
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - id: find-exercises
       run: |
         echo \
@@ -32,7 +32,7 @@ jobs:
       matrix:
         exercise: ${{ fromJson(needs.find-exercises.outputs.exercises) }}
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Check formatting for '${{ matrix.exercise }}'
       run: cargo fmt -- --check
       working-directory: exercism/rust/${{ matrix.exercise }}