Always derive from process.env when spawning a child process/shell execution

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.

editors/code/src/run.ts

@@ -177,7 +177,7 @@ export function createTask(spec: ra.Runnable): vscode.Task {
         label: spec.label,
         command: spec.bin,
         args: spec.extraArgs ? [...spec.args, '--', ...spec.extraArgs] : spec.args,
-        env: spec.env,
+        env: Object.assign({}, process.env, spec.env),
     };
 
     const execOption: vscode.ShellExecutionOptions = {