package/win32: build.sh: disable MSI generation on aarch64

We are using WIX 3.5 which doesn't have arm64 support. It was added in WIX 3.14
[1] but it doesn't have a stable release version yet [2].

Supporting newer WIX requires a significant amount of work (and a switch in our
Docker images).

[1] https://github.com/wixtoolset/issues/issues/5558
[2] https://wixtoolset.org/releases/