[migration] re-try without print_function when compiling a script fails
As part of the python3 porting we added the __future__.print_function import to
cubicweb.migration. That started affecting all migration scripts it runs,
which may not be prepared. So if compiling a script results in SyntaxError,
try again without the print_function flag.
Closes #7935030
[test] do not overwrite user's CW_CUBES_PATH in unittest_migration
Currently when you rely on your CW_CUBES_PATH environment variable to
locate cubes necessary for cubicweb's tests, the migration test crashes
because it overrides the env var.