Enable code signing on GH Actions (#18034)
Part of #17878
Summary of the issue:
Code signing is missing from GitHub actions, which needs to be added to have full parity to AppVeyor
Description of user facing changes
None
Description of development approach
Set in api signing token in GitHub secrets, create a signed build if appropriate
Testing strategy:
Ensure a try build based on this PR is signed: https://github.com/nvaccess/nvda/actions/runs/15868127863
