Add SCRITCHER_RUNNER env var

Closes #13

src/main.zig

@@ -104,8 +104,8 @@ pub fn doRepl(allocator: *std.mem.Allocator, args_it: var) !void {
     var runqs_args = langs.ArgList.init(allocator);
     defer runqs_args.deinit();
 
-    // TODO change the runqs command to something given in an env var
-    try runqs_args.append("ristretto");
+    const wanted_runner: []const u8 = std.os.getenv("SCRITCHER_RUNNER") orelse "ristretto";
+    try runqs_args.append(wanted_runner);
 
     while (true) {
         lang.reset();