Add CARGO_WORKSPACE_DIR env var in .cargo/config.toml

As discussed in rust-analyzer/expect-test#33

Author: fasterthanlime

.cargo/config.toml

@@ -6,3 +6,6 @@ lint = "clippy --all-targets -- -Aclippy::collapsible_if -Aclippy::needless_pass
 
 [target.x86_64-pc-windows-msvc]
 linker = "rust-lld"
+
+[env]
+CARGO_WORKSPACE_DIR = { value = "", relative = true }