[SBRP] Support creating new reference packages from a Windows environment

[MichaelSimons]

In order to provide a better UX for .NET developers, this repo should have support for creating new ref packs from a Windows environment. There are two phases of support needed.

1. Add ability to run the generate tooling from a Windows environment. Today the generate tooling is accessible only via a bash script.
2. Add ability to run the source-build leg from a Windows environment. This is important to be able to detect prebuilts locally. This will eventually be needed for .NET 9.0 as part of .NET 9: Enable source-build for Windows #2910.

The first phase as value and is something that can be delivered without the second phase.

[MichaelSimons]

cc @ViktorHofer

[ViktorHofer]

dotnet/source-build-reference-packages#581 covers the first point. I also attempted to add a build.cmd entry point to build the repository locally on Windows but the build then failed because of these Unix Exec tasks in the targeting pack generation:

• https://github.com/dotnet/source-build-reference-packages/blob/e1b6b6ae61a710e96f5278188379bb496318fae4/src/targetPacks/Directory.Build.targets#L18-L22
• https://github.com/dotnet/source-build-reference-packages/blob/e1b6b6ae61a710e96f5278188379bb496318fae4/src/targetPacks/Directory.Build.targets#L55-L58
• https://github.com/dotnet/source-build-reference-packages/blob/e1b6b6ae61a710e96f5278188379bb496318fae4/src/targetPacks/Directory.Build.targets#L62-L64