Normalise the JSON redirects file before diffing

This ensures that key-ordering changes, which don't affect the
semantics of the file, don't create diff noise.

.github/workflows/publish.yml

@@ -115,6 +115,28 @@ jobs:
           name: build-from-main
           path: main
 
+      - name: Normalise JSON
+        shell: python
+        run: |
+          import json
+          from pathlib import Path
+
+          ROOTS = [
+              Path('main/_site'),
+              Path('local/_site'),
+          ]
+          FILES = [
+              'redirects.json',
+          ]
+
+          for root in ROOTS:
+              for name in FILES:
+                  p = root / name
+                  p.write_text(json.dumps(
+                      json.loads(p.read_text()),
+                      sort_keys=True,
+                  ))
+
       - name: Diff
         id: diff
         run: |