Make npm upgrade non-fatal in CI setup for broken macOS runners

The macOS runner's pre-installed npm has a corrupted promise-retry
module, causing `npm install -g npm@11` to fail. Since all CI jobs
use yarn (not npm), this step is non-critical. Emit a warning
annotation and continue instead of failing the job.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: kieran-osgood-shopify

.github/actions/setup/action.yml

@@ -12,7 +12,7 @@ runs:
 
     # TODO: Can be removed once node-version is 24 as this will be the minimum
     - name: Install npm 11
-      run: npm install -g npm@11
+      run: npm install -g npm@11 || echo "::warning::npm upgrade failed — not required for yarn-based CI"
       shell: bash
 
     - name: Cache turbo build setup