As per #30645, fish with fish-foreign-env prints this
(harmless) warning:
```
set: Tried to change the read-only variable “_”
```
This patch was developed by @rnhmjoj in the aforementioned
issue discussion
* Patched fish to load /etc/fish/config.fish if it exists (by default,
it only loads config relative to itself)
* Added fish-foreign-env package to parse the system environment
closes#5331
