Hasn't been run since the travis days.
Now managed through a deploy key rather than an access token to limit scope to
a single repo. Private key is stored in qTox/qTox as a repository secret, and
the public portion is saved to qTox/doxygen as a deploy key.
Matches Windows and AppImage build script behaviour.
Irrelevant for CI runs, but since the build dir is inside the user's mounted
qTox dir, this stops manual runs from deleting a possibly existing directory,
and allows the user to chose the build dir.
Requires removing of the check for committed generated cmake files, but I can't
imagine those making it past review anyway.
