bi-directional env passing

## Expected Behavior
https://github.com/google/zx#passing-env-variables
As defined in the above URL, you can pass env variables from the script to shell commands. It would be nice to also do it the other way around like:
```ts
await $`source .envfile`
console.log(process.env) //containing the above sourced env variables
```

## Actual Behavior
Env variables are not passed to `process.env`