Issue parsing surrogate pair encoded emoji in pipeline JSON file

We're seeing an issue where the buildkite agent fails to parse the JSON-formatted pipeline file we're passing to `buildkite-agent pipeline upload`:

```
Pipeline parsing of "tmpjdd2BG" failed (Failed to parse tmpjdd2BG: line 531: found invalid Unicode character escape code)
```

I was able to reproduce the issue [on the Go Playground](https://go.dev/play/p/iyAetDay0rc) using the same YAML library that the agent uses. The Go standard library JSON parser handles the input without an issue.

I found [a related issue](https://github.com/go-yaml/yaml/issues/279) on the upstream go-yaml project.

Start by reproducing the failure through `buildkite-agent pipeline upload`, comparing the Go Playground example with the agent's pipeline parsing path and the related go-yaml issue. Done means a pipeline JSON file containing a surrogate-pair-encoded emoji parses successfully without the invalid Unicode escape error.

Written by the indexing model from the issue text.