Add the sign step to the windows build

[drew2a]

This PR adds the sign step to the windows build.

The certificate is stored as an environment secret base64 string.

See:

• https://docs.github.com/en/actions/security-guides/encrypted-secrets#creating-encrypted-secrets-for-an-environment
• https://docs.github.com/en/actions/security-guides/encrypted-secrets#storing-base64-binary-blobs-as-secrets

To continue the build procedure (and to get access to the secret), the approval of reviewers is needed. These reviewers are listed in the "Build" environment: https://github.com/Tribler/tribler/settings/secrets/actions

The windows build procedure will be the following (after merging to the main):