Tidy up building of `_img.zip` file

[petefoth]

Currently. the name of the infile for the call to img_from_target_files is hardcoded, and therefore slightly brittle: it will fail of LOS change the precise naming

infile="out/target/product/$codename/obj/PACKAGING/target_filesintermediates/lineage$codename-target_files-eng.root.zip"

Better to get the filename using find (as we do for img_dir later on

So change to

infile=$(find "$source_dir/out/target/product/$codename/obj/PACKAGING" -name "lineage_$codename-target_files*.zip")
echo ">> [$(date)] Making -img.zip file from $infile" | tee -a "$DEBUG_LOG"

[petefoth]

Also apply this change in my IodeBuild project