scripts/dashboard: kill existing xvfb process

scripts/dashboard: kill existing xvfb process

To fix the below error on starting cypress
```
[STARTED] Task without title.
[FAILED] Xvfb exited with a non zero exit code.
[FAILED]
[FAILED] There was a problem spawning Xvfb.
[FAILED]
[FAILED] This is likely a problem with your system, permissions, or installation of Xvfb.
[FAILED]
[FAILED] ----------
[FAILED]
[FAILED] Error: _XSERVTransSocketUNIXCreateListener: ...SocketCreateListener() failed
[FAILED] _XSERVTransMakeAllCOTSServerListeners: server already running
[FAILED] (EE)
[FAILED] Fatal server error:
[FAILED] (EE) Cannot establish any listening sockets - Make sure an X server isn't already running(EE)
[FAILED]
[FAILED] ----------
[FAILED]
[FAILED] Platform: linux-x64 (Ubuntu - 22.04)
[FAILED] Cypress Version: 12.17.4
Xvfb exited with a non zero exit code.

There was a problem spawning Xvfb.

This is likely a problem with your system, permissions, or installation of Xvfb.
```

Signed-off-by: Nizamudeen A <nia@redhat.com>