Revert "Fix missing variable expansion $ in psub"

That change was a bit too eager as the mkfifo route doesn't currently work.

See #1040 and #2052.

This reverts commit a17b9fdabaccb126b4e619b861cc914568616933.

share/functions/psub.fish

@@ -49,7 +49,7 @@ function psub --description "Read from stdin into a file and output the filename
 		set TMPDIR /tmp
 	end
 
-	if test $use_fifo = 1
+	if test use_fifo = 1
 		# Write output to pipe. This needs to be done in the background so
 		# that the command substitution exits without needing to wait for
 		# all the commands to exit