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/context/manager.rs

@@ -162,7 +162,13 @@ impl ContextManager {
     }
 
     /// Set current context, without modifying configuration
-    pub fn set_current_ephemeral(&self, context: Context) {
+    ///
+    /// Accepts either an owned [`Context`] or an already-shared
+    /// `Arc<Context>`: callers that keep their own handle to the context
+    /// they install (e.g. [`crate::deb::ephemeral::EphemeralContextGuard`])
+    /// pass the Arc so they can restore exactly this context afterwards
+    /// instead of relying on whatever happens to be current at that time.
+    pub fn set_current_ephemeral(&self, context: impl Into<Arc<Context>>) {
         *self.context.write().unwrap() = context.into();
     }