packaging: install and run Inno files in a build directory
Upcoming commits will refactor the Windows installers so their
content is dynamically derived. In preparation for this, we
copy the Inno files into a new build directory and run them
from there. This required changing some relative paths to
work from a new directory level but is otherwise a pretty
straightforward change.
Differential Revision: https://phab.mercurial-scm.org/D7157
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.