Hadrian resolves executables names to absolute paths unnecessarily

Currently hadrian/src/Builder.hs uses lookupInPath to lookup the full path of any tools in system.cfg on Windows (seemingly to ensure that the tool name has a .exe extension). However, two things:

• It calls lookupInPath on all operating systems, despite only (possibly) using the result on Windows
• A comment by @angerman rightly points out that this sort of resolution is unnecessary and undesireable: if the user provided a relative path then we should use a relative path, not underhandedly subvert their intention. If the tool name needs an extension then we should either make this so in the configure script or leave it to the user.