Update build.zig for Zig 0.13.0

[hans-tvs]

cwd_relative is relative to the cwd of the build runner process, which causes problems (missing files, as my build step looks in the wrong location for zig-protobuf's source code) when I try to import zig-protobuf as a module or add a proto-generating build-step using the exact instructions in the README.

This PR just replaces that with b.path, which makes those dependencies relative to your build.zig instead. I'm not intimately familiar with all your build steps, so it might be worth spot-checking that's semantically correct, but that change is sufficient for me to be able to import and use this in another Zig project in the manner described in the README.

[HendrikJanssen]

Interesting, I just now stumbled on that exact same issue!

I also think it would be nice to correct the zig-cache directory to the relocated .zig-cache dir in https://github.com/Arwalk/zig-protobuf/blob/9dd4eef382693296d7d7f3f8e35a76635aad4bdb/build.zig#L284

Also, the README.md should probably include the correct way to setup the gen-protoc step:

const protoc_step = protobuf.RunProtocStep.create(b, protobuf_dep.builder, target, .{
        .destination_directory = b.path("src/proto"),
        .source_files = &.{
            "protocol/all.proto",
        },
        .include_directories = &.{},
    });

[hans-tvs]

Nice catch. I was wondering where that zig-cache was coming from, and I forgotten I'd made that same destination_directory change in my calling code.

[Arwalk]

Thanks a lot, guys. I realise I haven't checked imports in an external project, so I'm not surprised this error appeared.

I'll check this ASAP. I am very happy to know people use this library, and your comments and commits are appreciated.