make: clean up job.c

There used to be a time when Job_Init only ran the commands of the
special .BEGIN target, but that's several years ago.

The 'becomes' for echoTmpl was confusing since it referred to a time
where echoTmpl shared the same memory location as errOn, which was
really confusing since echoTmpl, as the name says, is a printf template
while errOn is a plain string.  All this confusion, just to save 4 or 8
bytes of memory, was not worth it.

Since shellArgv is only used for freeing it and no code is supposed to
ever interpret that memory, rename the variable.

(rillig)