graph: Re-enable postponed index creation in debug builds

The v0.44.0 disable patch hardcoded `postpone_attribute_index_creation`
to `false`, which broke three `relational::ddl_tests` (`generate_ddl`,
`generate_postponed_indexes`, `postponed_indexes_with_block_column`)
that exercise the postponed-index DDL path and relied on the prior
`cfg!(debug_assertions)` auto-on for that coverage.

Switch the field to `cfg!(debug_assertions)` so:

- Release builds (production): postponement stays off regardless of
  env var, matching the intent of the original disable.
- Debug builds (CI + dev): postponement is on, so the existing tests
  exercise the code path and pass.

The `GRAPH_POSTPONE_ATTRIBUTE_INDEX_CREATION` env var stays inert
(parsed but unused, hence the existing `#[allow(unused)]`).

Author: incrypto32

graph/src/env/mod.rs

@@ -351,7 +351,7 @@ impl EnvVars {
             subgraph_error_retry_ceil: Duration::from_secs(inner.subgraph_error_retry_ceil_in_secs),
             subgraph_error_retry_jitter: inner.subgraph_error_retry_jitter,
             enable_select_by_specific_attributes: inner.enable_select_by_specific_attributes.0,
-            postpone_attribute_index_creation: false,
+            postpone_attribute_index_creation: cfg!(debug_assertions),
             postpone_indexes_creation_threshold: inner.postpone_indexes_creation_threshold,
             log_trigger_data: inner.log_trigger_data.0,
             explorer_ttl: Duration::from_secs(inner.explorer_ttl_in_secs),