Bump version to 6.4.0

[joshmgross]

Preparing to release v6.4.0 which will include node-fetch - #321

[github-actions]

Hello from actions/github-script! (ae926c4)

[maxkostow]

This should have been a breaking change for anyone that manually installed node-fetch

const fetch = require('node-fetch')

Will now throw an error

SyntaxError: Identifier 'fetch' has already been declared

[joshmgross]

Sorry about that @maxkostow, I'll keep that in mind for future releases.

At this point users are likely depending on node-fetch that's provided by this action, so I don't think we can roll back.