Flaky e2e test for the XGBoost model with the 'gblinear' booster

Now Go is failing (refer to #200 (comment)):

=================================== FAILURES ===================================
_ test_e2e[xgboost_XGBClassifier - go_lang - train_model_classification_binary2] _
estimator = XGBClassifier(base_score=0.6, booster='gblinear', colsample_bylevel=None,
              colsample_bynode=None, colsamp...ambda=0, scale_pos_weight=1, subsample=None,
              tree_method=None, validate_parameters=False, verbosity=None)
executor_cls = <class 'tests.e2e.executors.go.GoExecutor'>

...

expected=[0.04761511 0.9523849 ], actual=[0.047615, 0.952385]
expected=[0.06296992 0.9370301 ], actual=[0.06297, 0.93703]
expected=[0.12447995 0.87552005], actual=[0.124479, 0.875521]
expected=[0.0757848 0.9242152], actual=[0.075784, 0.924216]
expected=[0.8092151  0.19078489], actual=[0.809212, 0.190788]

BTW, in attempts to check my guess from #200 (comment), I found that coefs in gblinear are also float32:
https://github.com/dmlc/xgboost/blob/67d267f9da3b15a6e5a8393afae9be921a4e224b/src/gbm/gblinear_model.h#L110

https://github.com/dmlc/xgboost/blob/67d267f9da3b15a6e5a8393afae9be921a4e224b/src/gbm/gblinear_model.h#L120

https://github.com/dmlc/xgboost/blob/67d267f9da3b15a6e5a8393afae9be921a4e224b/src/gbm/gblinear_model.h#L82

https://github.com/dmlc/xgboost/blob/67d267f9da3b15a6e5a8393afae9be921a4e224b/src/gbm/gblinear_model.h#L91

and from #188 (comment) we know that bst_float is actually float
https://github.com/dmlc/xgboost/blob/8d06878bf9b778db68ae98f68d99a3557c7ea885/include/xgboost/base.h#L110-L111

Created dmlc/xgboost#5634.