OK, so core20 (1015) bundles libc6 2.31-0ubuntu9.3 which has been removed from updates. Please build a new core20 with libc6 2.31-0ubuntu9.2 which is currently in focal-updates.
OK, so core20 (1015) bundles libc6 2.31-0ubuntu9.3 which has been removed from updates. Please build a new core20 with libc6 2.31-0ubuntu9.2 which is currently in focal-updates.