VS Code: Options parameter in tasks.json not able to change env variables?

Question

I have the following tasks.json file with the options declared in the windows section

{
  "version": "2.0.0",
  "tasks": [
    {
      "label": "Bazel Build (Debug)",
      "type": "shell",
      "command": "bazel  build --cxxopt='-std=c++17' ${relativeFileDirname}:${fileBasenameNoExtension} -c dbg",
      
      "windows": {
        "options": {
          "env": {
            "relativeFileDirname": "src/main" 
          }
        },
        "command": "bazel build --cxxopt='-std=c++17' ${relativeFileDirname}:${fileBasenameNoExtension} --enable_runfiles -c dbg --verbose_failures"
      }
    }
   ]
 }

I am trying to change the relativeFileDirname environment paramter to src/main from src\main as Bazel is still looking for unix like directory separators, even on windows.

How can I get the options to change the environment variable for the windows build task only?