[BUG] Global variables not accessible/modifiable when falling back to slow path

**Describe the bug**
```python
In [1]: %load_ext cudf.pandas

In [2]: lst = []

In [3]: def udf(x):
...: lst.append(x)
...: return x
...:

# "ValueError: user defined function compilation failed." in cudf
In [4]: pd.Series(range(2)).apply(udf)
Out[4]:
0 0
1 1
dtype: int64

In [5]: lst
Out[5]: []

In [6]: import pandas

In [7]: pandas.Series(range(2)).apply(udf)
Out[7]:
0 0
1 1
dtype: int64

In [8]: lst
Out[8]: [0, 1]
```