Fixes bug introduced in commit 54150029.

Using script run-time filename does not always work. One cannot assume
that the filename is always the same as there might be temporary file
names used by dpkg in certain situations. See Debian #920415.

The same fix has been successfully in use in Debian official packages
since February 2019:
https://salsa.debian.org/mariadb-team/mariadb-10.3/commit/6440c0d6e75