Add a missing [workspace] tag in rust-gbt Cargo to build in git

If cargo detects its being run in a git tree, it looks for a
top-level `Cargo.toml`. When failing to find one, it errors out,
saying "current package believes it's in a workspace when it's not."

Instead, we add a `[workspace]` tag to let cargo know that rust-gbt
is not, in fact, in a rust workspace.

backend/rust-gbt/Cargo.toml

@@ -6,6 +6,8 @@ authors = ["mononaut"]
 edition = "2021"
 publish = false
 
+[workspace]
+
 [lib]
 crate-type = ["cdylib"]