Currently the list of trusted expressions grows indefinitely, and we
keep gc roots to the latest build for every file we ever
nix-buffer into. This can be manually cleaned up by nuking the state
dir and clearing nix-buffer--trusted-exprs, but this should be
handled more automatically.
Right now nix-buffer will almost certainly fail for remote
files, unless f.el and call-process somehow conspire to do the
right thing here.