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