guix-install.sh: Gracefully fail on | bash.

* etc/guix-install.sh (welcome): Print an error message and a hint if
the first read fails.

etc/guix-install.sh

@@ -615,7 +615,10 @@ https://www.gnu.org/software/guix/
 EOF
     # Don't use ‘read -p’ here!  It won't display when run non-interactively.
     echo -n "Press return to continue..."$'\r'
-    read -r char
+    if ! read -r char; then
+	echo
+	die "Can't read standard input.  Hint: don't pipe scripts into a shell."
+    fi
     if [ "$char" ]; then
 	echo
 	echo "...that ($char) was not a return!"