deb: pass the build context explicitly instead of swapping the global

build_binary_package installed its ephemeral chroot context into the
process-global manager and read it back with context::current(),
ignoring its ctx parameter: two concurrent builds would re-point each
other's global and each drop would clean up whichever chroot was
current at the time. The guard now keeps the Arc of the context it
created (parented directly on the base context, not on a config-name
lookup), exposes it via context(), and Drop cleans up exactly that
context and restores the exact handle that was current at creation,
so overlapping builds no longer cross-destroy each other.

src/deb/mod.rs

@@ -134,14 +134,17 @@ async fn build_binary_package_impl(
         None
     };
 
-    let result = async {
-        // Get the build context - either the ephemeral context or the base context
-        let build_ctx = if mode == BuildMode::Local {
-            context::current()
-        } else {
-            base_ctx.clone()
-        };
+    // Determine the build context explicitly: for Local builds it is the
+    // ephemeral context the guard just created (taken from the guard itself,
+    // never from the process-global, which concurrent builds may have
+    // re-pointed at their own chroot); otherwise the base context is used
+    // directly.
+    let build_ctx = match guard.as_ref() {
+        Some(g) => g.context(),
+        None => base_ctx.clone(),
+    };
 
+    let result = async {
         // Prepare build directory
         let build_root = build_ctx.create_temp_dir()?;