`target_dir` parameter does not resolve VS Code variables

[achary]

Extension version: 0.7.8 VS Code version: 1.53.1 OS: Linux Ubuntu 20.04

When settings.json gets:

"rust.target_dir": "${workspaceFolder}/my-cache/rls"

then it looks like the variable name ${workspaceFolder} is not resolved to a directory path and the extension creates directories named literally '${workspaceFolder}/my-cache/rls' in workspace crates. This seems like a deficiency, as it's possible to provide here an absolute path to the parameter, and it seems like the extension uses it correctly to put all background build files into this single directory (which may very well be outside of crates locations etc).