lib/std/os/uefi/protocol/file.zig: change io adapter fns to const ptr

[dotcarmen]

The rest of the methods in this type expect a *const File, and the type definitions even specify *const File. It's also only possible to construct a *const File ie

const uefi = @import("std").uefi;
const fs = try boot_services.openProtocolSt(uefi.protocol.SimpleFileSystem, uefi.handle);
var vol: *const uefi.protocol.File = undefined;
try fs.openVolume(&vol).err();

where fs.openVolume expects a **const File. As such, calling vol.reader() results in a type error, since a *const File can't be downcasted to *File

[dotcarmen]

tbh, the rest of the API could use some consistency 😅 for example BlockIo's methods use *BlockIo instead of *const BlockIo. I can follow-up this pr with additional corrections if desired

[linusg]

The rest of the methods in this type expect a *const File

I believe all of these are incorrect - even though it's fine to treat self as a const pointer from the Zig compiler's perspective, this only works because the UEFI API function pointers are also (incorrectly) typed this way - they definitely get mutated in the underlying implementation (seek offset for instance). Being able to read and write from a *const File is nonsense, so I'd rather not paper over it like this.