CI: properly enable the string dtype also for custom CI builds

Author: jorisvandenbossche

.github/workflows/unit-tests.yml

@@ -91,7 +91,7 @@ jobs:
       LANG: ${{ matrix.lang || 'C.UTF-8' }}
       LC_ALL: ${{ matrix.lc_all || '' }}
       PANDAS_CI: '1'
-      PANDAS_FUTURE_INFER_STRING: ${{ matrix.pandas_future_infer_string || '0' }}
+      PANDAS_FUTURE_INFER_STRING: ${{ matrix.pandas_future_infer_string || '1' }}
       TEST_ARGS: ${{ matrix.test_args || '' }}
       PYTEST_WORKERS: 'auto'
       PYTEST_TARGET: ${{ matrix.pytest_target || 'pandas' }}