Automatically GPG-sign the commits made by @jlbuild

[DilumAluthge]

It would be great if @jlbuild could automatically GPG-sign the commits it makes when it makes a PR to the General registry.

This would require the following steps:

1. Generate a new GPG keypair
2. Upload the GPG public key to the GitHub account for @jlbuild
3. Copy the GPG private key to the appropriate server
4. Configure Yggradasil/BinaryBuilder.jl to sign all commits with the GPG private key