Ignore read-only variables
Ignore read-only variables

These variables cannot be set in fish:
https://github.com/fish-shell/fish-shell/blob/7f59a7e7cfd2d49d8df3d652ff1e1b8a0b1527bc/src/env.cpp#L83-L96
If a script exports this variable an error message will be emitted along these lines:

    set: Tried to change the read-only variable “_”

In this case it's particularly annoying becausethe underscore variable is [automatically set by bash](https://www.gnu.org/software/bash/manual/html_node/Special-Parameters.html#Special-Parameters):
| Also set to the full pathname used to invoke each command executed and placed in the environment exported to that command.
edc/bass,edc/bass