Add .NET SDK search paths to global.json

[ITaluone]

Adds the local .NET SDK install directories to global.json, so SDK resolution finds the version installed by the build scripts.

What changed

• Search .fallout/temp/dotnet-win and .fallout/temp/dotnet-unix before installed host SDKs.
• Keep $host$ as the fallback for system-wide SDK installations.

Verification

• Change required .NET version in global.json (and for testing from latestPatch to latestMinor) to a version neither installed on the system nor installed locally
• Solution keeps "unloading" or more precise fails to load anything
• run build.ps1 / build.sh which installs the required .NET version
• See the solution loads again automagically, even SDK is not installed on the system

Another way of verification:

• run dotnet --list-sdks or dotnet --version and see the local project paths listed

Fixes: #669