Make stdout linebuffered 

Since you redirect both stdout and stderr to a pipe and capture it together, both should be linebuffered to produce nice output.

Here is a PoC:

```lua
local ffi = require('ffi')

ffi.cdef([[
  typedef struct __IO_FILE FILE;
  void setlinebuf(FILE *stream);
  FILE *stdout;
  FILE *stderr;
]])

ffi.C.setlinebuf(ffi.C.stdout)
ffi.C.setlinebuf(ffi.C.stderr)
```