duckdb: non-literal date is no longer cast as date when converted from snowflake

[tekumara]

The following conversion fails when executed by duckdb, because DATE is not a valid duckdb expression:

❯ python -c 'import sqlglot; print(repr(sqlglot.parse_one("SELECT date(a.start_date) from SOURCE_TABLE AS a", read="snowflake").sql(dialect="duckdb")));'
'SELECT DATE(a.start_date) FROM SOURCE_TABLE AS a'

sqlglot 21.2 didn't have this problem, and would cast a non-literal as date, eg:

❯ python -c 'import sqlglot; print(repr(sqlglot.parse_one("SELECT date(a.start_date) from SOURCE_TABLE AS a", read="snowflake").sql(dialect="duckdb")));'
'SELECT CAST(a.start_date AS DATE) FROM SOURCE_TABLE AS a'