Apply @AndreyAkinshin's suggestion

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

@@ -148,10 +148,12 @@ private static ProcessStartInfo CreateStartInfo(BenchmarkCase benchmarkCase, Art
                     start.Arguments = args;
                     start.WorkingDirectory = artifactsPaths.BinariesDirectoryPath;
                     break;
-                default:
+                case CustomRuntime _:
                     start.FileName = exePath;
                     start.Arguments = args;
                     break;
+                default:
+                    throw new NotSupportedException("Runtime = " + runtime);
             }
             return start;
         }