GitHub Actions CI Windows jobs are unexpectadly unable to find package data in mayavi

The following failure was seen here - https://github.com/enthought/mayavi/runs/3776106273?check_suite_focus=true

```
...
File "C:\hostedtoolcache\windows\Python\3.6.8\x64\lib\site-packages\mayavi\core\file_data_source.py", line 199, in __set_pure_state__
raise IOError(msg)
OSError: Could not find file at D:\C:\hostedtoolcache\windows\Python\3.6.8\x64\lib\site-packages\mayavi\tests\data\cube.vti
```

Note the `"D:\"` at the beginning of the file path which could not be found - but the package/environment is in `"C:\"`.

This failure can be temporarily addressed by simply installing `mayavi` from source and not running the testsuite from a temporary directory.

_Originally posted by @rahulporuri in https://github.com/enthought/mayavi/pull/1076#discussion_r720669948_