do the build/test on Linux as well

Author: ericsink

.github/workflows/dotnet.yml

@@ -11,8 +11,7 @@ jobs:
 
     strategy:
       matrix:
-        # TODO want Linux here too, but the apple workloads aren't available there.
-        os: [windows-2025, macos-15]
+        os: [ubuntu-24.04, windows-2025, macos-15]
 
     runs-on: ${{ matrix.os }}