Cached offline use regression since lock file copying was implemented

With Rust analyzer version 0.3.2509 for VSCode I was unable to use my typical workflow for working offline.

Before #20018 was merged I was able to connect to the internet, run cargo metadata, and as long as my crate versions didn't change I was able to continue using it offline.

I'm on a restricted machine where the main GUI desktop can't connect to the internet, but I can open up a shell that has internet access and I tend to run cargo fetch in that shell to get everything downloaded before using the GUI offline.

The issue now is that rust-analzyer is creating a new folder at target/rust-analyzer/metadata and since I haven't run cargo metadata in that folder ( since it doesn't exist until running the GUI with an internet connection ) the metadata isn't cached and rust-analyzer can't startup.

I realize that this is a little bit of a special case, but it still feels like maybe I should be able to have an option to run rust-analyzer against the normal cargo metadata location.

Originally posted by @zicklag in #12499

Reproduce the offline workflow described in the issue: run cargo fetch and cargo metadata, then compare the normal metadata location with target/rust-analyzer/metadata. Trace how rust-analyzer chooses that metadata directory. Done means the cached offline workflow can start without requiring metadata to be generated in the newly created directory.