Currently the list of trusted expressions grows indefinitely, and we
keep gc roots to the latest build for every file we ever
nix-buffer-enter 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-enter will almost certainly fail for remote
files, unless f.el and call-process somehow conspire to do the
right thing here.