Use short name for DebianArm64 (#28264)

src/Testing/src/xunit/HelixConstants.cs

@@ -6,6 +6,6 @@ namespace Microsoft.AspNetCore.Testing
     public static class HelixConstants
     {
         public const string Windows10Arm64 = "Windows.10.Arm64v8.Open;";
-        public const string DebianArm64 = "(Debian.9.Arm64.Open)[email protected]/dotnet-buildtools/prereqs:debian-9-helix-arm64v8-a12566d-20190807161036;";
+        public const string DebianArm64 = "Debian.9.Arm64.Open;";
     }
 }