Fix shell injection in update-changelog workflow for PR titles with backticks (#897)

PR titles containing backticks (e.g., "Remove `v` prefix from release
tags") cause the changelog workflow to fail with `v: command not found`
because bash interprets backticks as command substitution.

### Changes

- Move variable assignments from `run:` to `env:` block in the "Update
CHANGELOG.md" step

### Before
```yaml
run: |
  PR_TITLE="${{ github.event.pull_request.title }}"  # Backticks interpreted by bash
```

### After
```yaml
env:
  PR_TITLE: ${{ github.event.pull_request.title }}  # Set by GHA before shell runs
run: |
  ./bin/update-changelog.sh "$PR_TITLE" ...
```

Using the `env:` block ensures GitHub Actions sets environment variables
before the shell executes, preventing interpretation of special
characters.

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Author: Copilot

.github/workflows/update-changelog.yml

@@ -58,13 +58,13 @@ jobs:
           echo "Determined category: $CATEGORY"
 
       - name: "Update CHANGELOG.md"
+        env:
+          PR_TITLE: ${{ github.event.pull_request.title }}
+          PR_NUMBER: ${{ github.event.pull_request.number }}
+          CATEGORY: ${{ steps.category.outputs.category }}
+          GITHUB_REPOSITORY_URL: "https://github.com/${{ github.repository }}"
         run: |
-          PR_TITLE="${{ github.event.pull_request.title }}"
-          PR_NUMBER="${{ github.event.pull_request.number }}"
-          CATEGORY="${{ steps.category.outputs.category }}"
-
           # Use the standalone script to update the changelog
-          export GITHUB_REPOSITORY_URL="https://github.com/${{ github.repository }}"
           ./bin/update-changelog.sh "$PR_TITLE" "$PR_NUMBER" "$CATEGORY"
 
       - name: "Commit and push changes"