contrib: correct default build path in man pages

The gen-manpages.py script is unable to find the executables because it tries to search in /bitcoin/src when no custom BUILDDIR is set. With the new build process in place, the default build dir is now set to /bitcoin/build.

Prior to this change, the script stopped with a not found or not an executable error. Now it generates the man pages without needing to explicitly set BUILDDIR variable.