Only churn rows once in test_scrubber_physical_gc_ancestors (#10481)

## Problem

PR #10457 was supposed to fix the flakiness of
`test_scrubber_physical_gc_ancestors`, but instead it made it even more
flaky. However, the original error causes disappeared, now to be
replaced by key not found errors.

See this for a longer explanation:
https://github.com/neondatabase/neon/issues/10391#issuecomment-2608018967

## Solution

This does one churn rows after all compactions, and before we do any
timeline gc's. That way, we remain more accessible at older lsn's.

test_runner/regress/test_storage_scrubber.py

@@ -271,8 +271,14 @@ def test_scrubber_physical_gc_ancestors(neon_env_builder: NeonEnvBuilder, shard_
         ps.http_client().timeline_compact(
             shard, timeline_id, force_image_layer_creation=True, wait_until_uploaded=True
         )
-        # Add some WAL so that we don't gc at the latest remote consistent lsn
-        workload.churn_rows(1)
+
+    # Add some WAL so that we don't gc at the latest remote consistent lsn
+    workload.churn_rows(10)
+
+    # Now gc the old stuff away
+    for shard in shards:
+        ps = env.get_tenant_pageserver(shard)
+        assert ps is not None
         ps.http_client().timeline_gc(shard, timeline_id, 0)
 
     # We will use a min_age_secs=1 threshold for deletion, let it pass