[Merge] ~juliank/grub/+git/bionic:master into ~ubuntu-core-dev/grub/+git/bionic:master

[Julian Andres Klode]

Yes, the grub-check-signatures would be nicer with traps, but this is the exact code that's in all other stable series, not introducing regression potential now.
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https://code.launchpad.net/~juliank/grub/+git/bionic/+merge/436705
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