use 3.1.300-preview to fix embedded pdb (#1161)

Rx.NET/Source/global.json

@@ -1,5 +1,8 @@
 {
   "msbuild-sdks": {
     "MSBuild.Sdk.Extras": "2.0.54"
+  },
+  "sdk": {
+    "version": "3.1.300-preview"
   }
 }

azure-pipelines.rx.yml

@@ -33,10 +33,11 @@ stages:
       DOTNET_SKIP_FIRST_TIME_EXPERIENCE: true
 
     steps:
-    - task: UseDotNet@2
+    - task: DotNetCoreInstaller@0
       inputs:
-        version: '3.1.x'
+        version: '3.1.300-preview-015048'
         performMultiLevelLookup: true
+        includePreviewVersions: true
 
     - task: DotNetCoreCLI@2
       inputs: