Update checked-in build definition to fix tests

Checked in definition contains 'master' which ends up causing race condition in tests.  Set HelixBranch to actual branch used.

Author: MattGal

buildpipeline/Dotnet-CoreClr-Trusted-BuildTests.json

@@ -205,7 +205,7 @@
       "value": "coreclr"
     },
     "HelixBranch": {
-      "value": "master"
+      "value": "release/2.0.0"
     },
     "HelixApiAccessKey": {
       "value": null,
@@ -294,4 +294,4 @@
     "state": "wellFormed",
     "revision": 418097633
   }
-}
+}