Strip the windows installer files
Summary:
This has been lost during the switch from Gitian to GUIX.
This reduces the size of the windows installer from ~168MiB to ~25MiB.
Test Plan: Run the windows GUIX build, check the installer and binary run properly on windows.
Reviewers: #bitcoin_abc, roqqit
Reviewed By: roqqit
Differential Revision: https://reviews.bitcoinabc.org/D16598