stdenv: steal `checkInputs` from buildPythonPackage

Note that a bunch of non-python packages use this attribute already.
Some of those are clearly unaware of the fact that this attribute does
not exists in stdenv because they define it but don't to add it to
their `bulidInputs` :)

Also note that I use `buildInputs` here and only handle regular
builds because python and haskell builders do it this way and I'm not
sure how to properly handle the cross-compilation case.