Adjust .report.legacy per PRICE_EMISSION_NEW

[khaeru]

I've just tried to run pytest -k test_legacy.

This fails with message_ix main, which is expected.
It also fails with the branch from iiasa/message_ix#726 as of right now:

message_ix_models/tests/report/test_legacy.py:37:                              
    report(test_context) 
…
message_ix_models/report/legacy/pp_utils.py:1867: in _retr_crb_prc                                                                                             
    df = ds.var(var, {"type_tec": ["all"]})                                                                                                                    
…
RuntimeError: unhandled Java exception: There exists no variable 'PRICE_EMISSION_NEW'!

This seems to be because recent changes on that branch have removed this variable.

This likely means that the above test will begin to fail within our scheduled workflow. We will need to adjust the code for this change. Some options:

• Wait until changes are make downstream, in iiasa/message_data:ssp_dev or elsewhere. (I guess in the interim this code will not be using the same message_ix GAMS code as in #726, but something else? Unclear.)
• Make some manual adjustment directly to message_ix_models.report.legacy.