Due to the way the paths initial_rustc and initial_cargo were
constructed before this commit, they mixed \ and / for path separators
and they omitted the .exe suffix.

This worked fine up until now, as Windows is capable of handling the
mixed path separators and the Command::new API adds the ".exe" suffix if
missing from the executable.

This resulted in paths that didn't actually exist on disk though, due to
the missing .exe suffix. This commit fixes that by adding the .exe
suffix to initial_rustc and initial_cargo when --build is Windows.