R package tasks accumulate env vars

[jennybc]

This bit of code causes any task-specific env vars to accumulate in env and therefore carry forward to all tasks later in the list:

positron/extensions/positron-r/src/tasks.ts

Lines 69 to 78 in 9c57708

	const env: any = {};
	const pandocPath = getPandocPath();
	if (pandocPath) {
	env['RSTUDIO_PANDOC'] = pandocPath;
	}
	return taskData.map(data => {
	if (data.envVars) {
	Object.assign(env, data.envVars);
	}

So far, none of the env vars for different tasks conflict with each other, so it's not doing any active harm. I discovered this incidentally while poking around these env vars in the debugger for other reasons. But I think it would be good to fix.