Before this PR

There were some cases where we'd still try to set _ unsuccessfully, e.g. when sourcing the per-user nix.sh file when running a Nix installation.
See #19 (comment)

With some debugging messages printed out as to what it was returning from fenv.parse.diff:

Setting: NIX_SSL_CERT_FILE=/etc/ssl/certs/ca-certificates.crt
Setting: PATH=/Users/dansanduleac/.nix-profile/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin
Setting: _=/Users/dansanduleac/.nix-profile/bin/env
set: Tried to change the read-only variable “_”

After this PR

Always filter out _ from being set in fish.

Actually fixes what #19 tried to achieve.