deb: add -j/--jobs to control parallel build jobs

By default the number of parallel jobs is detected with nproc inside
the build context. Add a -j/--jobs option so an explicit count can be
honored instead, threading it through build_binary_package and
local::build into DEB_BUILD_OPTIONS=parallel=N.

src/deb/mod.rs

@@ -33,6 +33,7 @@ pub async fn build_binary_package(
     inject_packages: Option<&[&str]>,
     ctx: Option<Arc<Context>>,
     ui: Option<Arc<DebUi>>,
+    jobs: Option<usize>,
 ) -> Result<Vec<PathBuf>, Box<dyn Error>> {
     let result = build_binary_package_impl(
         arch,
@@ -45,6 +46,7 @@ pub async fn build_binary_package(
         inject_packages,
         ctx,
         &ui,
+        jobs,
     )
     .await;
 
@@ -68,6 +70,7 @@ async fn build_binary_package_impl(
     inject_packages: Option<&[&str]>,
     ctx: Option<Arc<Context>>,
     ui: &Option<Arc<DebUi>>,
+    jobs: Option<usize>,
 ) -> Result<Vec<PathBuf>, Box<dyn Error>> {
     let cwd = cwd.unwrap_or_else(|| Path::new("."));
 
@@ -160,6 +163,7 @@ async fn build_binary_package_impl(
                     inject_packages,
                     build_ctx.clone(),
                     ui.clone(),
+                    jobs,
                 )
                 .await?
             }
@@ -414,6 +418,7 @@ mod tests {
             None,
             Some(ctx),
             None,
+            None,
         )
         .await
         .expect("Cannot build binary package (deb)");