starship: fix fish integration syntax

The previous fish integration for starship erroneously used parts of
POSIX-esque test syntax. It also used `-n` instead of `-z` to check
for an unset variable.

PR #1422

modules/programs/starship.nix

@@ -101,7 +101,7 @@ in {
     '';
 
     programs.fish.promptInit = mkIf cfg.enableFishIntegration ''
-      if test [[  "$TERM" != "dumb"  -a \( -n "$INSIDE_EMACS"  -o "$INSIDE_EMACS" == "vterm" \)  ]]
+      if test "$TERM" != "dumb"  -a \( -z "$INSIDE_EMACS"  -o "$INSIDE_EMACS" = "vterm" \)
         eval (${cfg.package}/bin/starship init fish)
       end
     '';