Adapt the data import wizard to use the original $HOME on Linux

We change $HOME on Linux for various reasons, but this prevents the import procedure from working, plus maybe other disadvantages.

While exploring this might require more time (we have mullvad-browser#170 for it), we can probably adjust the paths Firefox uses for importing the data to use a (different) environment variable.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information