build scripts fail when using environment variable CARGO_BUILD_TARGET_DIR and setting rust-analyzer.cargo.extraEnv in settings.json

I'm trying to redirect check on save and rust analyzer builds to another location, however I've met some problems along the way

This is a two-fold bug report:
1: build scripts fail as the title says
2: cargo check fails without an explicit path (cannot use env var as I have)

Why it matters?
Saving on SSD usage by redirecting all builds to a ramdrive

For example, my project here, which sets the build dir to a unique ramdrive path (the reason why I have to use dynamic variables, so I can keep things in sync between vscode and powershell)
https://github.com/MolotovCherry/Cargo-RamDrive

Build scripts fail when setting rust-analyzer.cargo.extraEnv to env var in settings.json

When setting the following in settings.json

    "rust-analyzer.cargo.extraEnv": {
        "CARGO_BUILD_TARGET_DIR": "${env:CARGO_BUILD_TARGET_DIR}"
    },
    "rust-analyzer.checkOnSave.extraEnv": {
        "CARGO_BUILD_TARGET_DIR": "R:\\some\\path"
    },

Rust analyzer fails on startup with

failed to run build scripts Check the server logs for additional info

Note: VsCode was started in a shell with CARGO_BUILD_TARGET_DIR already set.
The contents of CARGO_BUILD_TARGET_DIR point to a valid directory, for example R:\Temp\rust\project-2140558008

The log error reads

[ERROR rust_analyzer::lsp_utils] failed to run build scripts

error: The directory name is invalid. (os error 267) at path "c:\\Users\\myusername\\Desktop\\rustfolder\\${env:CARGO_BUILD_TARGET_DIR}stX7G8"

Workaround

Disable build scripts in settings
"rust-analyzer.cargo.buildScripts.enable": false,

It's also worth noting that removing these configs and just starting VsCode with CARGO_BUILD_TARGET_DIR already set also works. Even so, I still think this is likely a bug.

Cargo check fails when using env var

Cargo check also fails if you use "${env:CARGO_BUILD_TARGET_DIR}" as the value. Not sure why it does

    "rust-analyzer.checkOnSave.extraEnv": {
        "CARGO_BUILD_TARGET_DIR": "${env:CARGO_BUILD_TARGET_DIR}"
    },

Workaround

Use explicit path;
It's also worth noting that removing these configs and just starting VsCode with CARGO_BUILD_TARGET_DIR already set also works. Even so, I still think this is likely a bug.

---

rust-analyzer version: rust-analyzer version: 0.3.1309-standalone (a2beeb8db 2022-12-04)

rustc version: rustc 1.67.0-nightly (e75aab045 2022-11-09)

relevant settings: N/A / see above

Reproduce the issue with rust-analyzer.cargo.extraEnv and rust-analyzer.checkOnSave.extraEnv in VS Code, using CARGO_BUILD_TARGET_DIR as shown. Start by tracing how these settings are passed to build scripts and cargo check; done means both operations accept the environment-variable value without invalid paths.