Specify node.js version in Github Actions example

[j0nas]

Specify node.js version for actions/setup-node@v1 step in example Github Actions code. Without this, the snippet currently fails as it defaults to an earlier version of node.js which raises a SyntaxError for the current version of the module.

[google-cla]

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[j0nas]

@googlebot I signed it!

[patrickhulce]

thanks @j0nas!