systemd-serialgetty: Replace sed quoting using ' with " to allow var expansion
A recent commit added single quotes around the sed regex's. This prevented the
expansion of $default_baudrate in do_install(), and ended up with systemd's'
serial-getty@.service file having a literal $default_baudrate.
This broke the serial console getty service.
serial-getty@.service was getting a line which looked like this:
