Enable Artifact Signing on macOS and Linux

[ellahathaway]

This issue was originally encompassed in #58445. I am moving it to this new issue for better clarity and tracking of the work that needs to be done.

Currently, aspnetcore builds on mac & linux, then shuttles those bits to Windows to sign using Arcade's SignTool. Now that there is support for signing directly on mac & linux with SignTool, aspnetcore builds should sign the artifacts on the build machines (eg if an artifact was built on mac, we should/can now sign it on a mac). In other words, we should alter aspnetcore to be more 'vertical'.

[ellahathaway]

This work is currently blocked. See #59590 (comment).

@wtgodbe - we spoke offline earlier about building aspnetcore on alpine via cross building it. Can you give me some more context on why you'd prefer to build aspnetcore, at the repo-level, on alpine vs cross building (eg in the VMR we use mcr.microsoft.com/dotnet-buildtools/prereqs:azurelinux-3.0-net9.0-cross-arm-alpine for a musl build)?

[wtgodbe]

Can you give me some more context on why you'd prefer to build aspnetcore, at the repo-level, on alpine vs cross building (eg in the VMR we use mcr.microsoft.com/dotnet-buildtools/prereqs:azurelinux-3.0-net9.0-cross-arm-alpine for a musl build)?

As a general principle, I prefer building on the target OS rather than cross-building for it, in case there are unexpected/arcane target-specific build errors. If we think the Microbuild work is going to be delayed longer than we expected, I'm fine with cross-building in the interim

[ellahathaway]

Signing has been enabled in the VMR, and aspnetcore will ship via the VMR once unified build is completed. As a result, this issue is no longer something that needs to be addressed.