add functions directory to provided constants

in https://github.com/netlify/build/blob/4d6202b0a141fdcd0d98335dc64d165c51bac0b4/packages/build/src/plugins/child/constants.js we provide cache dir build dir and config path but not functions dir. we should provide functions dir.

pseudocode i lifted from the functions plugin

	const { build } = config

    if (!build || !build.publish) {
      throw new Error('No build settings')
    }

    const functionsDir = build.functions
    if (!functionsDir) {
      console.log('No functions directory found')
      return
    }

    const buildDir = resolve(build.publish)
    const buildDirFunctions = resolve(buildDir, functionsDir)

[sw-yx]

destination i guess

but also source should build in the default value of functions for me so that i dont have to remember tbh

[ehmicky]

Done at #429.