Use DUNE_BUILD_DIR to search for files

[maxim092001]

Hi.

In input.ml we use _build as a hardcoded value for search. However, some users may override DUNE_BUILD_DIR or pass it as a flag to dune as per dune docs.

This issue is a request to use DUNE_BUILD_DIR instead of _build. Note, that DUNE_BUILD_DIR=_build by default.

Let me know if this change would make sense and you'd be happy for us to contribute it!

Best, Max.