rebuild-amount: Fix 'SC2155 (warning): Declare and assign separately to avoid masking return values.'

maintainers/scripts/rebuild-amount.sh

···
     # could eat too much memory for a standard 4GiB machine.
     local -a list
     for i in 1 2; do
-        local l="$($MKTEMP)"
+        local l
+        l="$($MKTEMP)"
         list[$i]="$l"
         toRemove+=("$l")
 
-        local expr="$($MKTEMP)"
+        local expr
+        expr="$($MKTEMP)"
         toRemove+=("$expr")
         nixexpr "${!i}" > "$expr"