[R][Wasm] Fix Error: thread constructor failed: Not supported under Wasm

### Describe the bug, including details regarding any error messages, version, and platform.

When I run `read_parquet` in a Wasm environment, I get,

> Error: thread constructor failed: Not supported

and the call returns a `data.frame` despite being called with `as_data_frame = FALSE`.

```r
> library(arrow)
> penguins <- read_parquet("/data/penguins.parquet", as_data_frame = FALSE)
Some features are not enabled in this build of Arrow. Run `arrow_info()` for more information.

Attaching package: ‘arrow’

The following object is masked from ‘package:utils’:

timestamp

Error: thread constructor failed: Not supported
```

### Component(s)

R