Add mkinitcpio support

[laniakea64]

Prior to #460 , system76-power required update-initramfs exclusively. Using system76-power on Arch Linux, which uses mkinitcpio for initramfs management, required the following patch:

https://aur.archlinux.org/cgit/aur.git/tree/use-mkinitcpio.patch?h=system76-power&id=0f49ee5b0bcda527abf55f5783e695528a9295d0

This PR builds on #460 and the above-linked AUR patch to add support for mkinitcpio without removing support for other initramfs management tools.

This PR is draft because:

• I'm not sure how to reach this code path and test this change? If it requires a System76 system with hybrid graphics, I have a gaze15 I could test this on if given instructions how to test.
• Although it compiles in Rust 1.89, Clippy throws a lot of warnings that are unrelated to these changes?

[laniakea64]

Looks like this PR will be obsoleted by #463 . Close this PR?