gdbserver: cleanup in handle_v_run

After the previous commit there is now a redundant string copy in
handle_v_run, this commit cleans that up.

There should be no functional change after this commit.

During review I was pointed to this older series:

  https://inbox.sourceware.org/gdb-patches/20211022071933.3478427-1-m.weghorn@posteo.de/



which also includes this fix as part of a larger set of changes.  I'm
giving a Co-Authored-By credit to the author of that original series.
I believe this smaller fix brings some benefits on its own, though the
original series does offer additional improvements.  Once this is
merged I'll take a look at rebasing and resubmitting the original series.

Co-Authored-By: Michael Weghorn <m.weghorn@posteo.de>
Approved-By: Tom Tromey <tom@tromey.com>