extendEnv does not work when combined with shell: true

[ehmicky]

The extendEnv option does not work as intended when combined with the shell: true option:

process.env.TEST = 'test'
const result = execa.sync('echo $TEST', { shell: true, env: {}, extendEnv: true })
assert.equal(result.stdout, 'test')

stdout is an empty string but should be 'test'

I will work on a PR to fix this.

[ehmicky]

I've opened a PR at #184