zsh: don't export HISTFILE and friends

Just set them normally.
Exporting them will propagate them to all executed programs
such as bash (as used by nix-shell or nix run),
and badness ensues when different formats are used.

nixos/modules/programs/zsh/zsh.nix

@@ -189,9 +189,10 @@ in
 
         . /etc/zinputrc
 
-        export SAVEHIST=${toString cfg.histSize}
-        export HISTSIZE=${toString cfg.histSize}
-        export HISTFILE=${cfg.histFile}
+        # Don't export these, otherwise other shells (bash) will try to use same histfile
+        SAVEHIST=${toString cfg.histSize}
+        HISTSIZE=${toString cfg.histSize}
+        HISTFILE=${cfg.histFile}
 
         ${optionalString (cfg.setOptions != []) "setopt ${concatStringsSep " " cfg.setOptions}"}