Limit xdist workers to 2 for engine-tests-docker

GHA standard runners have ~7GB RAM. Spark's JVM OOMs with the default
worker count. Limiting to 2 workers keeps memory usage in check.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: eakmanrq <6326532+eakmanrq@users.noreply.github.com>

Author: eakmanrq

.github/workflows/pr.yaml

@@ -254,6 +254,7 @@ jobs:
         engine:
           [duckdb, postgres, mysql, mssql, trino, spark, clickhouse, risingwave]
     env:
+      PYTEST_XDIST_AUTO_NUM_WORKERS: 2
       SQLMESH__DISABLE_ANONYMIZED_ANALYTICS: '1'
       UV: '1'
     steps: