[bug] launch.json envFile not capable of variable interpolation

Environment

• OS and version: Windows NT x64 10.0.19045
• VS Code: 1.99.1
• C/C++ extension:
• OS and version of remote machine (if applicable): N/A
• GDB / LLDB version: N/A

Bug Summary and Steps to Reproduce

Bug Summary:
.env files don't appropriately handle variable substitution in conformance with the dotenv file spec

Initially raised as https://github.com/microsoft/vscode/issues/257570 but advised to move the issue here.
Relates to #9329

Steps to Reproduce:

1. Create a blank repo and populate it with the following files:
   • launch.json

   {
       "version": "0.2.0",
       "configurations": [
           {
               "name": "(msvc) Launch",
               "type": "cppvsdbg",
               "request": "launch",
               "stopAtEntry": false,
               "cwd": "${workspaceFolder}",
               "environment": [],
               "envFile": "${workspaceFolder}/.env"
           }
       ]
   }
   

   • .env

   USER=admin
   EMAIL=${USER}@example.org
   PATH=~/project:${PATH}
   

2. Press F5 to trigger launch.json
3. Observe the following error message:

Invalid variable reference ${USER} in string: ${USER}@example.org.
Invalid variable reference ${PATH} in string: ~/project:${PATH}.

Expected behaviour

• USER=admin is exported to the environment
• EMAIL=admin@example.org is exported to the environment
• PATH=/home/username/project:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin (or some approximation thereof) is exported to the environment

Which would be consistent with Interpolation/Variable Substitution in dotenv files

Context

I'm trying to identify a transparent integration with conan for C++ development. One of the considerations for this is being able to update the following environment variables to allow the debugger to correctly resolve the paths to dependent libraries:

• LD_LIBRARY_PATH
• DYLD_LIBRARY_PATH
• PATH

Conan achieves this currently (unrelated to VSCode) by generating a conanrun.{sh,bat,ps1} set of files that updates the environment. The idea is that one does the following:

source build/Debug/generators/conanrun.sh
cmake --presets conan-release
cmake --build conan-release

It doesn't seem like VSCode has the ability to do the first part of this with the existing file formats as part of its application launch cycle. https://github.com/conan-io/conan/issues/11042 proposes the addition of an ability to generate a conanrun.env, which could be consumed by the envFile parameter. However, as discussed in https://github.com/conan-io/conan/issues/11042#issuecomment-3105696577 and https://github.com/conan-io/conan/issues/11042#issuecomment-3109473501, the syntax/features that VSCode expects/supports seems inconsistent to what is understood to be the dotenv file schema

Debugger Configurations

Above

Debugger Logs

Above

Other Extensions

Persists after disabling all other extensions.

Additional Information

No response

Reproduce the issue using the mentioned launch.json and .env files, then trace how the envFile parameter is processed when F5 starts the cppvsdbg configuration. Done means USER, EMAIL, and PATH substitutions are exported consistently with the stated expected behavior on the relevant platforms.