Don't fail with newer Debos versions

Debos has learned to not pollute images with spurious /etc/resolv.conf files, so the workaround now can break the build when trying to delete a file that is not there.

==== Drop resolv.conf so the appropriate one gets created at runtime ====
rm "${ROOTDIR}/etc/resolv.conf" | rm: cannot remove '/scratch/root/etc/resolv.conf': No such file or directory
Action `Drop resolv.conf so the appropriate one gets created at runtime` failed at stage Run, error: exit status 1

Use rm -f so the workaround keeps working with older Debos versions but does not break with newer ones.