Provide necessary entry points for environment scan in `read_duckdb_sql()`

``` r
con <- DBI::dbConnect(duckdb::duckdb(environment_scan = TRUE))
duckplyr::ducksql("FROM mtcars", con = con)
#> Error in `duckplyr::ducksql()`:
#> ! use of NULL environment is defunct
DBI::dbDisconnect(con)
```

<sup>Created on 2024-12-16 with [reprex v2.1.1](https://reprex.tidyverse.org)</sup>