Building etc."fish/setEnvironment.fish" needs
config.system.build.setEnvironment, which can be very large. And what
babelfishTranslate does is to translate env vars exported by bash
syntax, which does not need much computing power.
This patch can reduce the network traffic when using remote builders
with almost no harm.
A shared exported guard `__NIXOS_SET_ENVIRONMENT_DONE` is introduced that can
be used to prevent child shells from sourcing `system.build.setEnvironment`
the second time.
This fixes e.g. `nix run derivation` when run from e.g. ZSH through the console or
ssh. Before this Bash would resource the common environment resetting the `PATH`
environment variable.
We also export `system.build.setEnvironment` to `/etc/set-environment` making it
easy to reset the common environment with `. /etc/set-environment` when
needed and to grep for environment variables in `/etc` (which was the
motivation of #30418).
This reverts changes made in b00a3fc6fd
(the original #30418).
* 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
