tails-diff-suites: fix syntax for sorting on keys.

files/reprepro/tails-diff-suites

@@ -105,7 +105,7 @@ packages_in_iso() {
 sort_packages_list_inplace() {
    local list="$1"
 
-   sort --stable --key=1 --key=2 --output="$list" "$list"
+   sort --stable --key=1,1 --key=2,2 --output="$list" "$list"
 }
 
 ### Sanity checks