sandbox/_sandboxbwrap.py: Fix fuse import issue

Chandan Singhrequested to merge
chandan/fix-fuse-check into master

This will fix an issue where BuildStream currently throws an exception when running on Linux without fuse.

Fixes #1081 (closed).

I can verify that the tests for bst-plugins-container are green again with this change, as can be seen in: bst-plugins-container!46 (merged).

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