deps: upgrade npm to 6.10.1
Full release notes: https://npm.community/t/release-npm-6-10-1/8790
Notable changes:
- Fixes an EISDIR error in the cache
- Adds support for Visual Studio 2019 by upgrading node-gyp to 5.0.2
- Strip git environment variables when running git subcommands, so that running npm from within an existing git process does not get borked.