The [readthedocs build](https://readthedocs.org/projects/helmfile/builds/) is failing due to a missing required `build.os`
```
Error
Problem in your project's configuration. Invalid configuration option "build.os": build not found
```
Signed-off-by: Joseph Gardner <josephgardner@users.noreply.github.com>
Zubair Haque
Joseph Gardner
Yusuke Kuoka