NuGet requires that signed packages have a matching [registered signing
certificate](https://learn.microsoft.com/en-us/nuget/create-packages/sign-a-package#register-the-certificate-on-nugetorg).
Update release workflow to capture this certificate from the Sign CLI
tool and upload it as a release artifact. Note that this means we will
need to manually update this certificate to the
[`git-credential-manager`
organization](https://www.nuget.org/profiles/git-credential-manager) in
nuget.org prior to publishing the .NET tool version for each release.

Tested the end-to-end flow in [my
fork](https://github.com/ldennington/git-credential-manager) which
resulted in publication of [this
package](https://int.nugettest.org/packages/git-credential-manager) to
the NuGet QA Gallery.