Bug#1052690: grub2: post-install script overrides manual changes to GRUB_DISABLE_OS_PROBER

[Bertram Felgenhauer]

Sorry, I never tested whether the setting actually persists; I just
hoped that it would. But indeed it is not; I had to manually intervene
once again.

Jordi's analysis and patch look plausible to me. It might be slightly
better to compare "${GRUB_DISABLE_OS_PROBER}" to "true" instead of
"false" (swapping the branches of the conditional), since that's what
/etc/grub.d/30_os-prober does.

Thanks,

Bertram