Improve VSCode env var config substitution

[Veykril]

We currently only substitute ${env:ENV_VAR} in the env specific configs, ideally we should substitute these in the configs passed to the server in general.

Relevant code/FIXME

rust-analyzer/editors/code/src/config.ts

Line 275 in f1b257f

// FIXME: Merge this with `substituteVSCodeVariables` above