lxd/instance/drivers/driver/lxc: Close forkexec log file
lxd/instance/drivers/driver/lxc: Close forkexec log file

We only need this file for the initial launch of forkexec, the actual output from the requested command is via extra files.

Tested we still get launch failures in the forkexec.log file by modifying it to run as non-root to trigger the non-root check.

```
cmd.SysProcAttr = &syscall.SysProcAttr{
	Credential: &syscall.Credential{Uid: 1000},
}
```

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

lxc/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd