github/workflows: Update existing comments for code_size_comment.

This modifies the automated code size comment to edit an existing comment
if one already exists instead of always creating a new comment.  This
reduces noise on pull requests that are repeatedly updated.

Signed-off-by: David Lechner <david@pybricks.com>

.github/workflows/code_size_comment.yml

@@ -62,14 +62,44 @@ jobs:
           script: |
             const fs = require('fs');
 
-            github.rest.issues.createComment({
-              issue_number: Number(fs.readFileSync('pr_number')),
-              owner: context.repo.owner,
-              repo: context.repo.repo,
-              body: `Code size report:
+            const prNumber = Number(fs.readFileSync('pr_number'));
+            const codeSizeReport = `Code size report:
 
             \`\`\`
             ${fs.readFileSync('diff')}
             \`\`\`
-            `,
-            });
+            `;
+
+            const comments = await github.paginate(
+              github.rest.issues.listComments,
+              {
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                issue_number: prNumber,
+              }
+            );
+
+            comments.reverse();
+
+            const previousComment = comments.find(comment =>
+              comment.user.login === 'github-actions[bot]'
+            )
+
+            // if github-actions[bot] already made a comment, update it,
+            // otherwise create a new comment.
+
+            if (previousComment) {
+              await github.rest.issues.updateComment({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                comment_id: previousComment.id,
+                body: codeSizeReport,
+              });
+            } else {
+              await github.rest.issues.createComment({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                issue_number: prNumber,
+                body: codeSizeReport,
+              });
+            }