install.sh: use /dev/tty only when it can be opened

install.sh

@@ -39,7 +39,7 @@ for f in files:
 print(f"Downloaded {len(files)} files.")
 PY
 cd "$dest"
-if [ -r /dev/tty ] && [ -w /dev/tty ]; then
+if { : </dev/tty; } 2>/dev/null; then       # a real terminal: setup can ask for the sudo password
     bash setup.sh </dev/tty                  # piped from curl: setup's questions (sudo) read the terminal
 else
     bash setup.sh