Allow custom runtimes to use Executor (#2285)

* Allow custom runtimes to use Executor

* Apply @AndreyAkinshin's suggestion

Author: tgjones

src/BenchmarkDotNet/Environments/Runtimes/CustomRuntime.cs

@@ -0,0 +1,12 @@
+using BenchmarkDotNet.Jobs;
+
+namespace BenchmarkDotNet.Environments
+{
+    public abstract class CustomRuntime : Runtime
+    {
+        protected CustomRuntime(RuntimeMoniker runtimeMoniker, string msBuildMoniker, string displayName)
+            : base(runtimeMoniker, msBuildMoniker, displayName)
+        {
+        }
+    }
+}

src/BenchmarkDotNet/Toolchains/Executor.cs

@@ -148,6 +148,10 @@ private static ProcessStartInfo CreateStartInfo(BenchmarkCase benchmarkCase, Art
                     start.Arguments = args;
                     start.WorkingDirectory = artifactsPaths.BinariesDirectoryPath;
                     break;
+                case CustomRuntime _:
+                    start.FileName = exePath;
+                    start.Arguments = args;
+                    break;
                 default:
                     throw new NotSupportedException("Runtime = " + runtime);
             }