Update iso.sh

Issue:
I downloaded the 2 parts manually and then ran iso.sh.
iso.sh downloads the 2 parts again then create iso file.

Fix:
I'm adding a condition 
if the part exists already then it will skip downloading and then creates iso file,
else it will download the part that doesn't exist and then creates iso file.

.github/workflows/iso.sh

@@ -84,11 +84,16 @@ esac
 
 flavourcap=`echo ${flavour:0:1} | tr  '[a-z]' '[A-Z]'`${flavour:1}
 
-echo -e "\nDownloading ${flavourcap} ${ver}"
-echo -e "\nPart 1"
-curl -#L https://github.com/t2linux/T2-Ubuntu/releases/download/${latest}/${iso}.z01 > ${iso}.z01
-echo -e "\nPart 2"
-curl -#L https://github.com/t2linux/T2-Ubuntu/releases/download/${latest}/${iso}.zip > ${iso}.zip
+if [ ! -f ${iso}.z01 ]; then
+	echo -e "\nDownloading Part 1 for ${flavourcap} ${ver}"
+	echo -e "\n"
+	curl -#L https://github.com/t2linux/T2-Ubuntu/releases/download/${latest}/${iso}.z01 > ${iso}.z01
+fi
+if [ ! -f ${iso}.zip ]; then
+	echo -e "\nDownloading Part 2 for ${flavourcap} ${ver}"
+	echo -e "\n"
+	curl -#L https://github.com/t2linux/T2-Ubuntu/releases/download/${latest}/${iso}.zip > ${iso}.zip
+fi
 echo -e "\nCreating ISO"
 
 isofinal=$RANDOM