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

test/dotnet-build.Tests/GivenDotnetBuildBuildsProjects.cs

@@ -43,6 +43,26 @@ namespace Microsoft.DotNet.Tools.Builder.Tests
                 .Pass();
         }
 
+        [Fact]
+        public void It_builds_projects_with_ruleset_relative_path()
+        {
+            var testInstance = TestAssetsManager
+                .CreateTestInstance("TestRuleSet")
+                .WithLockFiles();
+
+            new BuildCommand(Path.Combine("TestLibraryWithRuleSet", "project.json"), skipLoadProject: true)
+                .WithWorkingDirectory(testInstance.TestRoot)
+                .ExecuteWithCapturedOutput()
+                .Should()
+                .Pass()
+                .And
+                .HaveStdErrContaining("CA1001")
+                .And
+                .HaveStdErrContaining("CA2213")
+                .And
+                .NotHaveStdErrContaining("CA1018"); // this violation is hidden in the ruleset
+        }
+
         [Fact]
         public void It_builds_projects_with_a_local_project_json_path()
         {