feat(users/picnoir/tvix-daemon): parse up to the operation

Using all the primitives recently implemented to nix-compat to reach
the point where the Nix client start to send us operation requests.

Using a small integration test script (or the VM test, but let's face
it, it's too slow to be useful), we manage to reach the point where
we're able to read a store operation:

    2024-03-21T18:53:27.624876Z  INFO tvix_daemon: Incoming connection addr=unix
    2024-03-21T18:53:27.625312Z  INFO worker:perform_init_handshake: tvix_daemon: Trust sent conn=Connection(unix) conn=Connection(unix)
    2024-03-21T18:53:27.625406Z  INFO worker: tvix_daemon: Client hanshake succeeded conn=Connection(unix)
    2024-03-21T18:53:27.625488Z  INFO worker: tvix_daemon: Operation received op=SetOptions conn=Connection(unix)

We had to take some shortcuts wrt. stderr/log management. The CPP Nix
codebase is a bit confusing in that area. I'll need to spend more time
reading this to fully understand what's happening there. For now,
sending the STDERR_LAST command to the client does the trick.

Change-Id: I9b0e20a52d885e64fe29188496aac5334de61edd
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11233
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/nix-compat/src/wire/worker_protocol.rs

@@ -6,6 +6,8 @@ use tokio::io::{AsyncReadExt, AsyncWriteExt};
 
 use crate::wire::primitive;
 
+pub static STDERR_LAST: u64 = 0x616c7473;
+
 /// Worker Operation
 ///
 /// These operations are encoded as unsigned 64 bits before being sent