Avoid trying to set new read-only variables

[keysmashes]

As of fish 4.8.0 (fish-shell/fish-shell@74ff7d2), there are a bunch of new read-only variables, which now causes a sea of errors after every prompt:

set: Tried to change the read-only variable '__fish_bin_dir'
set: Tried to change the read-only variable '__fish_cache_dir'
set: Tried to change the read-only variable '__fish_config_dir'
set: Tried to change the read-only variable '__fish_data_dir'
set: Tried to change the read-only variable '__fish_help_dir'
set: Tried to change the read-only variable '__fish_man_dir'
set: Tried to change the read-only variable '__fish_sysconf_dir'
set: Tried to change the read-only variable '__fish_user_data_dir'

This PR fixes that.