fix(tool): parse osrel sections without trailing null

[mjm]

In systemd 257.1, which is currently on staging-next in nixpkgs, systemd-boot no longer terminates its PE sections with null bytes: systemd/systemd#34950. This leads to an error when installing Lanzaboote:

Installing Lanzaboote to "/boot"...
Failed to read systemd-boot version from "/nix/store/qz2c1an9hyg1paah2xx6d1fdllq0lw09-systemd-257.1/lib/systemd/boot/efi/systemd-bootx64.efi".: Failed to parse C string.: data provided is not nul terminated
Failed to install bootloader

This change has us check the last byte of the section, expecting either a NUL or a newline, and then we create a &str from that accordingly.

I tested this on two machines: one with staging-next and one with nixos-unstable, and both were able to install Lanzaboote successfully.

[RaitoBezarius]

Tested manually.

[peigongdsd]

Will this patch get backported to v0.4.1?

[RaitoBezarius]

We should do a new minor release v0.4.2 to fix this bug.

[RaitoBezarius]

@nikstur @blitz could I have your assistance there?