Support buildOutputPath for Next.js

[khalil-omer]

buildOutputPath is not supported for OpenNext in NextjsSite like it is in the CDK version of SST. I tried pointing path to where the .next output is expected, but that doesn't work b/c it expects to find Next source code it seems, or at least it checks the path before running the build command.

[khalil-omer]

Update: this didn't end up being relevant for me, as I needed to provide my own build command, so I am able to use the actual OpenNext config. I'll leave this open as it would probably still help others.

[fwang]

Thanks @khalil-omer

Ion uses OpenNext v3. In v3, you can set buildOutputPath in your open-next.config.ts.