ci: Don't test web build on Windows

For unknown reasons, tests on the Windows web build continue to be
flaky on GitHub CI. Disable them for now. Tests continue to be run
on Ubuntu.

.github/workflows/test_web.yml

@@ -80,6 +80,8 @@ jobs:
         run: npm run lint
 
       - name: Run tests
+        # MIKE: Don't run web tests on Windows because it's flaky for unknown reasons. :-(
+        if: runner.os == 'Linux'
         working-directory: web
         run: npm test