make(1): move Job.xtraced to ShellWriter

This flag was placed wrong in the Job since it is only necessary as long
as the shell commands are written to the shell file.

Resetting it in JobStart and JobExec was completely misguided since that
is far away from writing the shell commands; this should have been done
in JobPrintCommands instead.

The status of this flag doesn't need to be printed in debugging mode
since it is controlled by a single command line option (-dx) and does
not interact with all the other switches.

(rillig)