tests: boot: Update imgtool path in west_sign_wrapper

[gchwier]

Fixes tests/boot/with_mcumgr in case, when imgtool is not installed. Added imgtool path to west sign wrapper.

Log from internal CI before fix:

INFO: CMD: west sign -t imgtool --no-hex --build-dir twister-out/nrf52840dk_nrf52840/zephyr/tests/boot/with_mcumgr/boot.with_mcumgr.test_upgrade/with_mcumgr --sbin twister-out/nrf52840dk_nrf52840/zephyr/tests/boot/with_mcumgr/boot.with_mcumgr.test_upgrade/test_0_0_2_0.signed.bin -- --key bootloader/mcuboot/root-rsa-2048.pem --version 0.0.2+0
WARNING: west sign using imgtool is deprecated and will be removed in a future release
FATAL ERROR: imgtool not found; either install it (e.g. "pip3 install imgtool") or provide --tool-path
FAILED

to test it one can run:
$ZEPHYR_BASE/scripts/twister -T tests/boot/with_mcumgr --device-testing --hardware-map hardware_map.yml --west-flash="--erase" --enable-slow -vv -c -ll debug

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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