Change ruby directory to 2.6.0.

Turns out I was testing the CL in an old mac machine but all the new
macs have catalina and ruby 2.6.0.

Change-Id: I7212c1ed89ac1280192d60a1460b99a5169f2c97
Bug: https://github.com/flutter/flutter/issues/65223
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/6482
Reviewed-by: Keyong Han <keyonghan@google.com>
Commit-Queue: Godofredo Contreras <godofredoc@google.com>