fix: validate.sh was incomplete for testing gitpod-io#23

`./validate.sh` currently fails on a fresh gitpod workspace with:

```
rm: cannot remove '/ide/xterm': Is a directory
ln: failed to create symbolic link '/ide/xterm/xterm': File exists
```

Author: erikmd

validate.sh

@@ -11,7 +11,7 @@ tar -xvf /workspace/rebuild/xterm.tar -C /workspace/rebuild/
 find /workspace/rebuild/ -name layer.tar -exec tar -xvf {} -C /workspace/rebuild/ \;
 
 cp /workspace/rebuild/ide/supervisor-ide-config.json /ide/
-rm /ide/xterm && true
+rm -fr /ide/xterm || true
 ln -s /workspace/rebuild/ide/xterm /ide/xterm
 echo "xterm: linked in /ide"