The entry may be already present for example when reinstalling package,
or calling the script multiple times (which apparently is the case
during system installation).
(cherry picked from commit 4d4e7cc5e9)
In non-EFI installation /boot/efi/EFI/qubes may not exists. In this case
do not try to touch (non-existing) files there.
FixesQubesOS/qubes-issues#1829
(cherry picked from commit f7eaa7bec2)
