[infra] Update find_base_commit.dart to default to main

This should fix the base builder.

Bug: b/198416551
Change-Id: I900a22f6e7d2f5b013de35a00103c987cff5a605
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/213042
Reviewed-by: Dmitry Stefantsov <[email protected]>
Commit-Queue: Alexander Thomas <[email protected]>

Author: athomas

tools/bots/find_base_commit.dart

@@ -22,7 +22,7 @@ void main(List<String> args) async {
   parser.addOption("branch",
       abbr: "B",
       help: "Select the builders building this branch",
-      defaultsTo: "master");
+      defaultsTo: "main");
   parser.addOption("count",
       abbr: "c", help: "List this many commits", defaultsTo: "1");
   parser.addFlag("help", help: "Show the program usage.", negatable: false);