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MINOR: [R] Replace use of show_query in test-duckdb.R with dbplyr::sql_build (#40955)

### Rationale for this change

I just ran the R package tests and saw a printed query mixed amongst testthat output:

```
[ FAIL 0 | WARN 0 | SKIP 1 | PASS 15 ]<SQL>
SELECT *
FROM arrow_010
```

I thought it would be good to silence this in some way.

### What changes are included in this PR?

I silenced this by changing out the call to `dplyr::show_query` for `dbplyr::sql_build` which produces different output but (1) doesn't print as as side-effect, (2) is specifically made for testing, and (3) still produces output we can use in this test.

For reference, this is what show_query prints (assuming via `cat`):
```
> show_query(table_four)
<SQL>
SELECT *
FROM arrow_011
```

Whereas `sql_build`:
```
> dbplyr::sql_build(table_four)
<dbplyr_table_ident[1]>
[1] `arrow_011`
```

### Are these changes tested?

Yes, but just manually on my system.

### Are there any user-facing changes?

No.

Authored-by: Bryce Mecum <petridish@gmail.com>
Signed-off-by: Bryce Mecum <petridish@gmail.com>