This is useful when an extension (e.g. Nix Environment Selector) or launch
configuration sets one or more environment variables.
When `env` is not explicitly specified in the options passed to
`child_process.spawn()` or `vscode.ShellExecution()`, then `process.env` gets
applied automatically. But when an explicit `env` is set, it should inherit from
`process.env` rather than replace it completely.