Fix MigrationDetails.currently_executing on Windows
Fix MigrationDetails.currently_executing on Windows

Because Windows' path include a `:` for the drive, the use of partition would split the path at the wrong place.
This uses index with a 2nd parameter to find the end of the path while skipping the starting part.
testdouble/good-migrations,testdouble/good-migrations,testdouble/good-migrations,testdouble/good-migrations