Cannot Set Squirrel env var SQUIRREL_TEMP

Looks as if spawn is never sent the opts argument, thus there seems to be no ability to set the SQUIRREL_TEMP env var. Was planning on opening a pull request, but with a variety of ways to handle, wondering if maintainers have a preference...?

1. Set single value directly in options:
`options.squirrelTempDir = 'xxxxx' -> spawn(cmd, args, {env: {SQUIRREL_TEMP: 'xxxxx'}})`

2. Provide an open ended env vars option
`options.squirrelEnvVars = {SQUIRREL_TEMP: 'xxxxx'} -> spawn(cmd, args, { env: options.squirrelEnvVars || null })`

Was initially leaning towards 2 if only because use of an env var for this, in and of itself seems 'temporary'. That said, any preference?