Fix false-positive check for tty in flatpak

When there is absolutely no tty set at all (e.g. in a shell spawned by
a flatpak), tty prints 'not a tty', which matches `tty | grep "tty"`.

Luckily `tty` also returns `1`, so we can do a two-stage check instead.

This fixes #38

conf.d/fishline-init.fish

@@ -13,7 +13,11 @@ end
 source $FLINE_THEME_DIR/default_symbols.fish
 
 # Load default color theme depending on terminal capabilities
-if begin; [ (uname) != "Darwin" ]; and tty | grep tty > /dev/null; end
+if begin;
+		[ (uname) != "Darwin" ]
+		and set -l out (tty)
+		and string match -eq "tty" $out;
+	end
     # Emable a TTY safe default theme if a TTY is detected
 	source $FLINE_THEME_DIR/tty_compatible.fish
 else if begin; not command -s tput > /dev/null; or [ (tput colors) -lt 256 ]; end