Execute 'csc' with working directory set to the project directory.

When using a ruleset with a relative path in buildOptions, csc can't
find the file because it is not working in the same directory as the
project.

Fix #2710

TestAssets/TestProjects/TestRuleSet/TestLibraryWithRuleSet/Violations.cs

@@ -0,0 +1,26 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System;
+using System.IO;
+
+namespace TestLibrary
+{
+    public class AttributeWithoutUsage : Attribute
+    {
+    }
+
+    public class ClassWithUndisposedStream
+    {
+        private Stream _nonDisposedStream = new MemoryStream();
+
+        public ClassWithUndisposedStream()
+        {
+        }
+
+        public Stream GetStream()
+        {
+            return _nonDisposedStream;
+        }
+    }
+}