image: fix pcr 12 calculation (#1706)

Kernel cmdline embedded in UKIs had no null terminator before. With newer versions of mkosi, it is already null-terminated so we shouldn't null terminate it twice.

image/measured-boot/precalculate_pcr_12.sh

@@ -23,9 +23,8 @@ cmdline_measure() {
   local path="$1"
   local tmp
   tmp=$(mktemp)
-  # convert to utf-16le and add a null terminator
+  # convert to utf-16le
   iconv -f utf-8 -t utf-16le "${path}" -o "${tmp}"
-  truncate -s +2 "${tmp}"
   sha256sum "${tmp}" | cut -d " " -f 1
   rm "${tmp}"
 }