fix(libsync): add OWNCLOUD_BULK_UPLOAD env override to force-disable bulk upload

Mirrors the existing OWNCLOUD_CHUNKING_NG override pattern in the same
file. Bulk upload (POST .../dav/bulk) is enabled automatically whenever
the server advertises the dav.bulkupload capability, with no
client-side way to opt out if that endpoint is broken or blocked
somewhere in the network path (reverse proxy, WAF, etc).

Observed against a self-hosted Nextcloud instance where every single
bulk-upload POST failed immediately with UnknownNetworkError, while
regular per-file uploads succeeded without issue. Because the failure
is deterministic and the client re-attempts bulk upload from scratch
on every sync invocation (the in-memory per-file blacklist does not
survive across the max-sync-retries restarts, let alone separate
nextcloudcmd invocations), affected files never converge and the sync
stays permanently red for that server.

Setting OWNCLOUD_BULK_UPLOAD=0 lets an operator route around a broken
bulk endpoint without needing a custom build, matching the escape hatch
already available for chunking via OWNCLOUD_CHUNKING_NG.

Author: mosandlt

src/libsync/capabilities.cpp

@@ -250,6 +250,11 @@ int Capabilities::maxConcurrentChunkUploads() const
 
 bool Capabilities::bulkUpload() const
 {
+    static const auto bulkUploadEnv = qgetenv("OWNCLOUD_BULK_UPLOAD");
+    if (bulkUploadEnv == "0")
+        return false;
+    if (bulkUploadEnv == "1")
+        return true;
     return _capabilities["dav"].toMap()["bulkupload"].toByteArray() >= "1.0";
 }