Try building the whole thing with 3.1

.github/workflows/build-ilspy.yml

@@ -42,7 +42,7 @@ jobs:
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: '5.0.x'
+        dotnet-version: 3.1.x
 
     - name: Add msbuild to PATH
       uses: microsoft/setup-msbuild@v1.0.2

global.json

@@ -3,7 +3,7 @@
 		"MSBuild.Sdk.Extras": "3.0.23"
 	},
 	"sdk": {
-		"version": "5.0.100",
+		"version": "3.1.100",
 		"rollForward": "latestFeature"
 	}
 }