Fix windows cmd redirection in ff payloads.

Command output from firefox shells running on windows was broken because a newline ended up in the wrong spot. This fixes the shells.

#### Verification

- [ ] On any windows, get a shell with a firefox payload:

        msf> use exploits/multi/browser/firefox_xpi_bootstrapped_addon
        msf> set LHOST my.ip.addr

- [ ] Ensure you can run `ping` and `echo abc` in the shell as expected