fix(libstore): enable S3 uploads in curl-based implementation

S3 uploads were failing with "uploading to 's3://...' is not supported"
because the enqueueItem function only allowed uploads to HTTP(S) URLs.

Since the HttpBinaryCacheStore now handles S3 URLs and generates S3
URLs for upload requests, we need to allow S3 URLs in enqueueItem when
AWS CRT support is available. The S3-to-HTTPS conversion already
happens in the TransferItem constructor, so this change enables full
read/write support for S3 stores.

Fixes uploads for commands like:
  nix copy --to s3://bucket /nix/store/path

Author: lovesegfault

src/libstore/filetransfer.cc

@@ -856,7 +856,11 @@ struct curlFileTransfer : public FileTransfer
 
     void enqueueItem(std::shared_ptr<TransferItem> item)
     {
-        if (item->request.data && !hasPrefix(item->request.uri, "http://") && !hasPrefix(item->request.uri, "https://"))
+        if (item->request.data && !hasPrefix(item->request.uri, "http://") && !hasPrefix(item->request.uri, "https://")
+#if NIX_WITH_AWS_CRT_SUPPORT
+            && !hasPrefix(item->request.uri, "s3://")
+#endif
+        )
             throw nix::Error("uploading to '%s' is not supported", item->request.uri);
 
         {