tools renode: reset SIGN_ARGS.

[philljj]

Description

SIGN_ARGS is getting duplicate args.

Testing

$./silly_reproducer 
Building key tools
make[1]: Entering directory '/home/jordan/work/boot/src/tools/keytools'
Building signing tool
Building keygen tool
make[1]: Leaving directory '/home/jordan/work/boot/src/tools/keytools'
/home/jordan/work/boot/src/tools/keytools/sign --ml_dsa --sha256 --ml_dsa --sha256  \

$cat silly_reproducer 
#!/bin/bash
# Any of these will do it:
#  cp config/examples/sim.config .config || exit 1
#  cp config/examples/sim-lms.config .config || exit 1
cp config/examples/sim-ml-dsa.config .config || exit 1
make distclean && make clean
make keytools || exit 1
make test-sim-internal-flash-with-update V=1 2>&1 | grep "sign.*sha256.*sha256"