run: Treat jj run -j0 <command> as use all available cores

Waleed noticed this in #4021.

cli/src/commands/run.rs

@@ -59,9 +59,8 @@ pub fn cmd_run(ui: &mut Ui, command: &CommandHelper, args: &RunArgs) -> Result<(
     // 2. the amount of cores available.
     // 3. a single job, if all of the above fails.
     let _jobs = match args.jobs {
-        Some(0) => return Err(user_error("must pass at least one job")),
+        Some(0) | None => std::thread::available_parallelism().map(|t| t.into()).ok(),
         Some(jobs) => Some(jobs),
-        None => std::thread::available_parallelism().map(|t| t.into()).ok(),
     }
     // Fallback to a single user-visible job.
     .unwrap_or(1usize);