GVls Plugin: switch Builder main to GVls 20.1

Current Builder 41's behavior broke gvls plugin by no providing the current configuration, that has been fixed with next GVls 20.1 release.

While there are work to be done at #1552 (closed) this will help users to get a functional LSP Vala Server.

One GVls 20.1 is released I'll prepare a MR to fix this issue.