Overwrite $HOME in chrom* desktop files.

After realizing that chromecognito works for other users, I began to
[suspect](http://www.chromium.org/user-experience/user-data-directory)
that `$XDG_CACHE_HOME` may be at fault. Overwriting `$HOME` made
chromecognito work for me again, even though I don't understand
why. Since these chrome sessions are meant to be single use anyway,
overwriting `$HOME` shouldn't be a problem though.