How can I prevent grub updates from resetting my EFI bootorder

sudo dpkg-reconfigure grub-efi-amd64

For the first couple of questions accept the existing values (just press Enter) but when it comes to this, select No:

 +----------------------| Configuring grub-efi-amd64 |-----------------------+
 |                                                                           |
 | GRUB can configure your platform's NVRAM variables so that it boots into  |
 | Debian automatically when powered on. However, you may prefer to disable  |
 | this behavior and avoid changes to your boot configuration. For example,  |
 | if your NVRAM variables have been set up such that your system contacts   |
 | a PXE server on every boot, this would preserve that behavior.            |
 |                                                                           |
 | Update NVRAM variables to automatically boot into Debian?                 |
 |                                                                           |
 |                    <Yes>                       <No>                       |
 |                                                ^^^^                       |
 +---------------------------------------------------------------------------+

The answer from Tobia worked for me on one machine, but on a different machine I found that grub-pc was installed instead of grub-efi-amd64. When I tried dpkg-reconfigure grub-pc, I found that it doesn't always ask this question, despite it sometimes being used in EFI mode. So getting to the crux of where this setting lives, I found that they are both actually modifying the debconf database entry named grub2/update_nvram. Thanks to this article, I found that you can directly set this variable with the following command:

echo "set grub2/update_nvram false" | debconf-communicate

You can confirm the setting this way:

echo "get grub2/update_nvram" | debconf-communicate

...which will output the current value of the variable.