bug: fish: variables set in `home.sessionVariables` are overwritten by `/etc/profile`

[hexagonal-sun]

Are you following the right branch?

• [X] My Nixpkgs and Home Manager versions are in sync

Is there an existing issue for this?

• [X] I have searched the existing issues

Issue description

I found an issue recently where $EDITOR wasn't being set, despite me setting it in home.sessionVariables. I tracked the issue down to the fact that /etc/profile provides a default value, which is set to nano by nixpkgs. When launching GNOME, /etc/profile is sourced by GNOME's startup stack and the variable is brought into the global environment. My fish.config ends up calling:

set -gx EDITOR "emacsclient"

But this ends up being ignored as the value sourced from /etc/profile takes precedent. This means setting some sessionVariables when using fish and GNOME will not work.

Maintainer CC

@octplane @Yure

System information

- system: `"x86_64-linux"`
 - host os: `Linux 6.7.0, NixOS, 24.05 (Uakari), 24.05.20240108.317484b`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.19.2`
 - channels(root): `"nixos"`
 - nixpkgs: `/nix/store/wckjmzcqc347li9r9154nvav82f22g8n-0g53xyh39z3y90p4d8r341wbqyjy1zhl-source`

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[PerchunPak]

Workaround I used

https://nixos.wiki/wiki/Command_Shell#Changing_the_default_shell