Enable signing on Non-Windows builds

[ellahathaway]

Related to dotnet/arcade#14431

Signing is now supported on Linux & Mac, so this PR enables signing directly in Mac & Linux builds. These changes also eliminate the need for the additional legs that used .azure/pipelines/jobs/codesign-xplat.yml@self to sign files generated by Linux and Mac builds.

[ellahathaway]

Leaving as a draft until I complete validation of these changes.

[ellahathaway]

mcr.microsoft.com/dotnet-buildtools/prereqs:cbl-mariner-2.0-cross-arm64-alpine, mcr.microsoft.com/dotnet-buildtools/prereqs:cbl-mariner-2.0-cross-arm-alpine, and mcr.microsoft.com/dotnet-buildtools/prereqs:alpine-3.19-WithNode will have to be updated to include apt and sudo. These are needed to install MicroBuild.

The issues here are that we 1) try not to include sudo in the dockerfiles, 2) alpine uses apk, and 3) the MicroBuild install task does not recognize when nuget is already cached. As a result, this is something that the MicroBuild team will have to adjust on their end.

This PR is blocked because of this issue.

[ellahathaway]

Given that this PR is currently blocked, I'm going to close it. I will reopen once it is unblocked (likely in a few weeks).