bug: `error: snippet id foo not defined` when not running with `-x`

[eamonburns]

Presenterm version: 0.16.1

Reproduction steps:
Create repro.md with the following contents:

```rust +exec +id:foo
fn main() {
    println!("hi");
}
```

<!-- snippet_output: foo -->

Run the following command:

presenterm repro.md

This is the error I got:

Error loading presentation:

error at repro.md:7:1:

  |
7 | <!-- snippet_output: foo -->
  | ^ snippet id 'foo' not defined

Note: This works:

presenterm -x repro.md