fix(iso): use underscore in volume label to comply with ISO 9660

ISO 9660 volume labels allow only A-Z, 0-9, and underscore.
Dashes cause xorriso WARNING on every build.
EASY-BEE-NVIDIA → EASY_BEE_NVIDIA (iso-application keeps dashes, it's UDF).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/builder/auto/config

@@ -30,7 +30,7 @@ lb config noauto \
     --linux-flavours "amd64" \
     --linux-packages "${LB_LINUX_PACKAGES}" \
     --memtest none \
-    --iso-volume "EASY-BEE-${BEE_GPU_VENDOR_UPPER:-NVIDIA}" \
+    --iso-volume "EASY_BEE_${BEE_GPU_VENDOR_UPPER:-NVIDIA}" \
     --iso-application "EASY-BEE-${BEE_GPU_VENDOR_UPPER:-NVIDIA}" \
     --bootappend-live "boot=live components video=1920x1080 console=tty0 console=ttyS0,115200n8 loglevel=7 username=bee user-fullname=Bee modprobe.blacklist=nouveau" \
     --apt-recommends false \