fix(nix): resolve stdenv deprecation warnings and gemini-cli deprecation

pkgs/applications/misc/volnoti/package.nix

@@ -1,7 +1,7 @@
 # Fix: Volnoti error: 'volnoti' has been removed due to lack of maintenance upstream.
 
 { pkgs, lib, ... }:
-if !pkgs.stdenv.isLinux then
+if !pkgs.stdenv.hostPlatform.isLinux then
   null
 else
 pkgs.stdenv.mkDerivation {

pkgs/slsk-batchdl/package.nix

@@ -6,7 +6,7 @@
   dotnetCorePackages,
 }:
 
-if !stdenv.isLinux then
+if !stdenv.hostPlatform.isLinux then
   null
 else
   buildDotnetModule rec {

pkgs/tools/misc/installer/package.nix

@@ -3,7 +3,7 @@
 # Inspired by https://github.com/cleverca22/nix-tests/blob/master/kexec/justdoit.nix
 # This script will partition and format drives; use at your own risk!
 
-if !pkgs.stdenv.isLinux then
+if !pkgs.stdenv.hostPlatform.isLinux then
   null
 else
   pkgs.writeShellScriptBin "installer" ''

pkgs/tools/misc/rebuild/package.nix

@@ -2,7 +2,7 @@
 
 pkgs.writeShellScriptBin "rebuild" ''
   echo ${pkgs.stdenv.hostPlatform.system}
-  SYSTEM=${if pkgs.stdenv.isDarwin then "darwin" else "linux"}
+  SYSTEM=${if pkgs.stdenv.hostPlatform.isDarwin then "darwin" else "linux"}
   if [ "$SYSTEM" == "darwin" ]; then
       sudo darwin-rebuild switch --flake ${builtins.toString ../../../../.}
   else