fix(libstore): restore S3 store registration

After removing the legacy S3 implementation in c833b26, the store
registration for s3:// URLs was accidentally lost. This caused commands
like `nix store info --store s3://bucket` to fail with "don't know how
to open Nix store with scheme 's3'".

This commit restores S3 support by:
1. Adding "s3" to HttpBinaryCacheStore's supported URI schemes when
   built with AWS CRT support
2. Fixing the S3-to-HTTPS URL conversion in TransferItem::init() to
   use the converted URL

The S3 functionality is now handled through the curl-based file transfer
layer with AWS SigV4 authentication, as intended by issue #13084.

Author: lovesegfault

src/libstore/filetransfer.cc

@@ -405,7 +405,14 @@ struct curlFileTransfer : public FileTransfer
                 curl_easy_setopt(req, CURLOPT_DEBUGFUNCTION, TransferItem::debugCallback);
             }
 
-            curl_easy_setopt(req, CURLOPT_URL, request.uri.c_str());
+            // Use the actual URL, which may have been transformed from s3:// to https://
+            std::string actualUrl = request.uri;
+#if NIX_WITH_AWS_CRT_SUPPORT
+            if (isS3Request && !result.urls.empty()) {
+                actualUrl = result.urls[0];
+            }
+#endif
+            curl_easy_setopt(req, CURLOPT_URL, actualUrl.c_str());
             curl_easy_setopt(req, CURLOPT_FOLLOWLOCATION, 1L);
             curl_easy_setopt(req, CURLOPT_MAXREDIRS, 10);
             curl_easy_setopt(req, CURLOPT_NOSIGNAL, 1);

src/libstore/http-binary-cache-store.cc

@@ -15,6 +15,10 @@ StringSet HttpBinaryCacheStoreConfig::uriSchemes()
     auto ret = StringSet{"http", "https"};
     if (forceHttp)
         ret.insert("file");
+#if NIX_WITH_AWS_CRT_SUPPORT
+    // S3 support is now handled via curl with AWS SigV4 authentication
+    ret.insert("s3");
+#endif
     return ret;
 }