Can't use non-root user with podSpecPatch

Hi, I'm trying to use the new functionality introduced with https://github.com/buildkite/agent-stack-k8s/pull/282 but running into some issues with checkout. I was initially seeing the checkout container error at startup with this message logged:
```
mkdir: can't create directory '//.ssh': Permission denied
```
I compared the resulting pod spec from before and after and noticed that previously [this logic](https://github.com/buildkite/agent-stack-k8s/blob/main/internal/controller/scheduler/scheduler.go#L582) seems to have been triggered to create a non-root user and it looks like that's no longer happening when using podSpecPatch.

I tried manually adding the root `securityContext` in my podSpecPatch but that causes other permissions errors after checkout, since it's trying to run as a non-root user later. It seems like the above linked logic needs to be triggered even when using podSpecPatch?