CI leg for community platform to avoid regressions #7677

Community platforms use cross-building through officially supported platforms to perform VMR end-to-end builds—e.g., freebsd-x64 cross-built on linux-x64. The most significant difference is that the dotnet/runtime build uses the --bootstrap flag for these platforms. This builds host tools in a first pass, then uses those live-built tools to fulfill requirements that typically come from nuget.org (see https://github.com/dotnet/dotnet/blob/0c4423f49f/src/runtime/docs/workflow/building/coreclr/cross-building.md#building-coreclr-with-bootstrapping).

This type of platform is not covered by the CI and runs into regressions from time to time: #398 (comment), #7596 etc. Once this "stage 0" SDK is built, we can perform a native build on the host machine using that stage 0 SDK. This is how the FreeBSD folks have been shipping for several years (cc @Thefrank).

The platforms which fall under this category are:

• freebsd-x64
• freebsd-arm64
• linux-loongarch64
• linux-riscv64
• illumos-x64
• solaris-x64
• haiku-x64
• openbsd-x64

Other platforms joining them in the near future are linux-s390x and linux-ppc64le (currently they are mono-only and coreclr work is in progress).

So we can see it is roughly a 50-50 split when it comes to officially-supported vs. community-supported platforms.

This is the generic build command for all of these platforms:
https://github.com/filipnavara/dotnet-riscv/blob/8abd76591d67cc23e77c6e01bf3a9b3cac207e90/.github/workflows/build.yml#L77-L80

It would be good to have one CI leg to exercise this use case using the same command, which takes 60-80 mins (well within the range of other CI builds). This is to prevent regressions earlier in the cycle. I recommend using freebsd-arm64 because:

1. The FreeBSD OS is the oldest and most mature in this category (the port started in 2015, though its arm64 architecture was added later).
2. It covers a "different OS" and "different arch" from the host OS (linux-x64); several conditions have had to be tweaked for them in the past.

MCR's docker tag for this one is azurelinux-3.0-net11.0-cross-freebsd-14-arm64 (soon it will be in azurelinux-4.0 as well: dotnet/dotnet-buildtools-prereqs-docker#1674).