binary_iso: Execute mkdir binary/.disk before writing to it

Before commit 9f3e5fe8 (Fix the way the .disk/mkisofs file is created) all these commands (mkdir, write to binary/.disk/mkisofs and xorriso) were in the same binary.sh script. Since that commit, the write was extracted, to prevent issues with quoting, but the related mkdir was left in binary.sh. This means that the write is now executed first, and the mkdir only afterwards, making the mkdir quite pointless.

In practice, this did not break becaue binary_disk also does the same mkdir and runs before binary_iso, but if one runs commands manually and skips binary_iso, then this does break.

Even though this is not really a supported usecase, just move the mkdir outside of binary.sh, so it runs before the write again as intended.