twister: footprints: Fix missing memory footprints

Commit 28b2e553 introduced a regression
into twister workflow, breaking memory footprint calculation.
The commit changed name of produced .elf files.
There is no more zephyr_prebuilt.elf and instead we have
zephyr_pre0.elf and zephyr_pre1.elf. This commit fixes a filter
telling which files to exclude when calculating memory footprints,
to match the current status.

Signed-off-by: Maciej Perkowski <Maciej.Perkowski@nordicsemi.no>