fix(libstore/filetransfer): prevent double callback invocation on interrupt during retry

Fix a race condition where interrupting a download (via Ctrl-C) during a
retry attempt could cause a crash. When `enqueueItem()` throws because the
download thread is shutting down, the exception would propagate without
setting `done=true`, causing the `TransferItem` destructor to invoke the
callback a second time.

This triggered an assertion failure in `Callback::rethrow()` with:
`Assertion '!prev' failed` and the error message `cannot enqueue download
request because the download thread is shutting down`.

The fix catches the exception from `enqueueItem()` and calls `fail()` to
properly complete the transfer, ensuring the callback is invoked exactly
once.

Author: lovesegfault

src/libstore/filetransfer.cc

@@ -598,7 +598,14 @@ struct curlFileTransfer : public FileTransfer
                     decompressionSink.reset();
                     errorSink.reset();
                     embargo = std::chrono::steady_clock::now() + std::chrono::milliseconds(ms);
-                    fileTransfer.enqueueItem(shared_from_this());
+                    try {
+                        fileTransfer.enqueueItem(shared_from_this());
+                    } catch (const nix::Error & e) {
+                        // If enqueue fails (e.g., during shutdown), fail the transfer properly
+                        // instead of letting the exception propagate, which would leave done=false
+                        // and cause the destructor to attempt a second callback invocation
+                        fail(std::move(exc));
+                    }
                 } else
                     fail(std::move(exc));
             }