FASTBuild: don't set ExecWorkingDir for custom command

We don't need set ExecWorkingDir since we add
a "cd" command to the script file anyway
(so the custom command will be executed in the correct directory).

Moreover, in 1.16 FASTBuild started using posix_spawn* API
instead of fork() (which is much more efficient),
but it only does so if `ExecWorkingDir` is not set.