enable msbuild server by default for team internal build pipelines (#11295)

### Part of #9379 

### Context
Enable MSBuildServer for our internal pipelines so that we can start
troubleshooting any issues that might arise.

### Changes Made
Enabled said server by toggling the appropriate environment variables.

### Testing
This is the test.

Author: SimaTian

.vsts-dotnet-ci.yml

@@ -167,6 +167,8 @@ jobs:
     inputs:
       filename: 'eng/cibuild_bootstrapped_msbuild.cmd'
       arguments: '-msbuildEngine dotnet -onlyDocChanged $(onlyDocChanged)'
+    env:
+      MSBUILDUSESERVER: "1"
   - task: PublishTestResults@2
     displayName: Publish .NET Framework Test Results
     inputs:
@@ -309,6 +311,8 @@ jobs:
         Token: $(dn-bot-dnceng-artifact-feeds-rw)
   - bash: . 'eng/cibuild_bootstrapped_msbuild.sh' --onlyDocChanged $(onlyDocChanged)
     displayName: CI Build
+    env: 
+        MSBUILDUSESERVER: "1"
   - task: PublishTestResults@2
     displayName: Publish .NET Test Results
     inputs:
@@ -374,6 +378,8 @@ jobs:
         Token: $(dn-bot-dnceng-artifact-feeds-rw)
   - bash: . 'eng/cibuild_bootstrapped_msbuild.sh' --onlyDocChanged $(onlyDocChanged)
     displayName: CI Build
+    env: 
+        MSBUILDUSESERVER: "1"
   - task: PublishTestResults@2
     displayName: Publish .NET Test Results
     inputs:

src/UnitTests.Shared/TestEnvironment.cs

@@ -56,6 +56,7 @@ public static TestEnvironment Create(ITestOutputHelper output = null, bool ignor
             // Clear these two environment variables first in case pre-setting affects the test.
             env.SetEnvironmentVariable("MSBUILDLIVELOGGER", null);
             env.SetEnvironmentVariable("MSBUILDTERMINALLOGGER", null);
+            env.SetEnvironmentVariable("MSBUILDUSESERVER", null);
 
             return env;
         }