So it happened again. After some random event like a system upgrade or power failure(?), what I see upon booting, somehow, is Windows Boot Manager instead of GRUB. Here’s how I fixed it.
First, let me be fair: Windows 10 didn’t really kill my GRUB, it just buried it alive. As far as I understand, what happened is that some EFI configuration was overwritten. GRUB was still there where it used to be, it just wasn’t being called during the boot process.
If I knew where my EFI GRUB executable was located, I could fix the problem right from a Windows command prompt (possibly as an Administrator):
bcdedit /set "{bootmgr}" path \EFI\grub_uefi\grubx64.efi
I am writing this note to my future self just to save me the trouble of finding out the \EFI\grub_uefi\grubx64.efi part again.
Anyway, I guess this location can change depending on the Linux distribution you are running (I am using Arch Linux in this particular machine), so here are some pointers: I booted from an old Linux bootable CD I had around, mounted my EFI partition and looked for grubx64.efi.
Not hard, but very, very annoying.