github/workflows: Comment on code size change instead of failing CI.

This changes the code size workflow to post a comment on pull requests with
the code size report.  It also removes the error threshold so that the test
won't fail if code size increases.

Allowable code size changes are subjective, so shouldn't cause CI to fail.
In addition, failing CI tests can cause other hooks like code coverage
reports to be suppressed, so this fixes that problem as well.

Fixes issue #8464.

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

.github/workflows/code_size.yml

@@ -24,4 +24,18 @@ jobs:
     - name: Build
       run: source tools/ci.sh && ci_code_size_build
     - name: Compute code size difference
-      run: tools/metrics.py diff --error-threshold 0 ~/size0 ~/size1
+      run: tools/metrics.py diff ~/size0 ~/size1 | tee > diff
+    - name: Save PR number
+      if: github.event_name == 'pull_request'
+      env:
+        PR_NUMBER: ${{ github.event.number }}
+      run: echo $PR_NUMBER > pr_number
+    - name: Upload diff
+      if: github.event_name == 'pull_request'
+      uses: actions/upload-artifact@v3
+      with:
+        name: code-size-report
+        path: |
+          diff
+          pr_number
+        retention-days: 1