Re-add build environment to mingw compiler probe
Re-add build environment to mingw compiler probe

Was removed in 6655d563c3, but the mingw compiler needs it to locate
some libraries.

Change-Id: I8b7d9ddb69a719c6a6ef590a680c89327cb360d5
Reviewed-by: Jake Petroules <2d627e1cecd5435752fa500bacbe05f7cc90c369@qt.io>

qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs