Previously execution of the trampoline would be somewhat sensitive to
GUILE_LOAD_PATH & co., for example.
* build-aux/build-self.scm (build-program): Remove 'unsetenv' call and
%LOAD-COMPILED-PATH hack.
(call-with-clean-environment): New procedure.
(with-clean-environment): New macro.
(build): Wrap 'open-pipe*' call in 'with-clean-environment'.