ci(fix): checkout before doing path filtering (vercel#9920)

### Description

We're seeing sporadic failures on the `dorny/paths-filter` part, and[
the recommended
recourse](dorny/paths-filter#24 (comment))
from the workflow author is to add a checkout.

Failures:
-
https://github.com/vercel/turborepo/actions/runs/13208878667/job/36878355246
-
https://github.com/vercel/turborepo/actions/runs/13205111177/job/36866322671

However, it's notable that it looks like GHA just...does this sometimes?
- actions/checkout#417
- actions/runner-images#9632
- actions/runner-images#9882

### Testing Instructions

CI

Author: anthonyshew

.github/workflows/turborepo-test.yml

@@ -26,6 +26,9 @@ jobs:
       with-tailwind-example: ${{ steps.filter.outputs.with-tailwind-example }}
       rest: ${{ steps.filter.outputs.rest }}
     steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
       - uses: dorny/paths-filter@v3
         id: filter
         with: