[guix] produce the NSIS installer when building for Windows

Summary:
and produce a .zip rather than a .tar.gz for windows, without forgetting .dll files (same procedure as gitian-win.yml)

and show the sha256 checksums at the end of the build

Test Plan:
HOSTS="x86_64-w64-mingw32 x86_64-linux-gnu" contrib/guix/guix-build

Check the content of the output directory.

Test the installer with wine

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15602