Fix Node 10 support

[ehmicky]

Array.flat() is not supported in Node 10:

netlify-plugin-inline-functions-env/index.js

Lines 39 to 40 in d4b0428

	const include = inputs.include && [inputs.include].flat()
	const exclude = inputs.exclude && [inputs.exclude].flat()

This could be fixed by using instead:

const include = inputs.include && [].concat(...[inputs.include])
const exclude = inputs.exclude && [].concat(...[inputs.exclude])

[bencao]

fixed with v1.0.8