WIP: Introduce an one-off-build command to reproduce ofBorg locally

Motivated by trying to review a change in how ofBorg reports build
errors I started hacking on a command that allowed me to execute just
the build part of ofBorg without setting up a local queue or config
files.

This is still a WIP that is hardcoded to exactly one PR and commit
revision. I plan to convert all the constants to Clap CLI arguments once
I moved the build system of the project to naersk and can then introduce
new dependencies.

Things to be done:

 - [ ] Move constants to Clap
   - [ ] The system attribute should be defaulted to the current system.
         We can use the `target_os` and `target_arch` values for that.
	 The user should still be able to override thos tho...
 - [ ] Make the logging a bit prettier
 - [ ] Implement a custom Notification receiver that just logs the
       messages as they come in.
 - [ ] (idea) add support for the eval part as well so we can run ofBorg
        locally just like we can run nixpkgs-review.

ofborg/src/bin/one-off-builder.rs

@@ -0,0 +1,70 @@
+/// Build attributes within a Nixpkgs PR the way that ofBorg does
+use ofborg::nix::Nix;
+use ofborg::notifyworker::SimpleNotifyWorker;
+use ofborg::tasks::build::BuildWorker;
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    ofborg::setup_log();
+    let current_system = "x86_64-linux";
+
+    let attrs = vec!["pulseview".to_owned()];
+
+    let nix = Nix::new(
+        current_system.to_owned(),
+        "daemon".to_owned(),
+        90 * 60,
+        None,
+    );
+    let p: std::path::PathBuf = "/tmp/ofborg".into();
+    let cloner = ofborg::checkout::cached_cloner(&p);
+    let worker = BuildWorker::new(
+        cloner,
+        nix,
+        current_system.to_owned(),
+        "one-off".to_string(),
+    );
+    let repo = ofborg::message::Repo {
+        owner: "nixos".to_owned(),
+        name: "nixpkgs".to_owned(),
+        full_name: "NixOS/nixpkgs".to_owned(),
+        clone_url: "https://github.com/nixos/nixpkgs.git".to_owned(),
+    };
+
+    let mut dummy_receiver = ofborg::notifyworker::DummyNotificationReceiver::new();
+
+    let pr = ofborg::message::Pr {
+        target_branch: Some("nixos-unstable".to_owned()),
+        number: 123170,
+        head_sha: "ec28b5d7a54fd1f8c2eba5e4ba238769d281755e".to_owned(),
+    };
+
+    let job = ofborg::message::buildjob::BuildJob::new(
+        repo,
+        pr,
+        ofborg::commentparser::Subset::Nixpkgs,
+        /* attrs */ attrs,
+        /* logs */ None,
+        /* statusreport: */ None,
+        /*request_id: */ "one-off".to_string(),
+    );
+
+    worker.consumer(&job, &mut dummy_receiver);
+
+    for message in dummy_receiver.actions {
+        use ofborg::worker::Action;
+        match message {
+            Action::Ack | Action::NackRequeue | Action::NackDump => println!("{:?}", message),
+            Action::Publish(msg) => match msg.content_type {
+                Some(x) if x == "application/json" => {
+                    let data: serde_json::Value = serde_json::from_slice(&msg.content).unwrap();
+                    println!("Action::Publish: {:?}", data);
+                }
+                _ => {
+                    println!("Action::Publish: {:?}", msg);
+                }
+            },
+        }
+    }
+
+    Ok(())
+}