installer: fix ability to use a custom OpenSslDir when building the Win32 installer.

The actual assignment was missing an "env." prefix.
installer: add the ability to use a custom bzip2 directory in the Win32 installer.

Previously, the bzip2.dll was always taken from the Ice directory.
This is because we previously used ZeroC's binary Ice releases, which
included a bzip2.dll that we could use.