fc-queue-runner: implement per-build cancellation via CancellationToken

Adds an `ActiveBuild` registry (DashMap of `<Uuid, CancellationToken>`)
to `WorkerPool` and get `dispatch()` to create a per-build token to race
`run_build` against it via Tokio's `select!`.

The `cancel_checker_loop` then polls the DB every N seconds (currently 2)
for builds cancelled while running, and triggers their tokens.

Existing `kill_on_drop(true) on `nix build` processes handles
subprocess cleanup when the future is dropped. Thank you past me for
your insight.

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: Ic8af58e92972c7d5d104d9c717e9217d6a6a6964

crates/queue-runner/Cargo.toml

@@ -11,6 +11,7 @@ anyhow.workspace             = true
 chrono.workspace             = true
 clap.workspace               = true
 config.workspace             = true
+dashmap.workspace            = true
 serde.workspace              = true
 serde_json.workspace         = true
 sqlx.workspace               = true