Improve interaction between `-x` and `-f` eval flags

When using eval's `-x` option, the `--format` option appears to be ignored.

Evaluating the following

```shell
pkl eval --format=json -x y - <Details

```pkl
x { 1; 2; 3 }
y { "a"; "b"; "c" }

output {
renderer = new JsonRenderer {}
value = y
}
```

Start with the eval command handling for the -x and --format options, using the supplied shell reproduction to observe which output path is selected. Compare the current result with the expected JSON array and verify that the fixed-format workaround remains unaffected.

Written by the indexing model from the issue text.