Update update script to automatically find CivicTheme directory

As this is to resolve an issue with the build script where it's unable to find the civictheme path, rather than hard-code a path in as part of the update script, we could resolve the path at runtime within the build script.

Example:

const DIR_CIVICTHEME            = getCivicthemeDir(PATH)

// ...

function getCivicthemeDir(subthemeDir) {
  let currentDir = subthemeDir
  while (currentDir !== path.parse(currentDir).root) {
    if (path.parse(currentDir).name === 'themes') {
      const civicthemePath = globSync(currentDir + '/**/civictheme').pop()
      if (fs.existsSync(civicthemePath) && fs.lstatSync(civicthemePath).isDirectory()) {
        return path.relative(civicthemePath, subthemeDir)
      }
    }
    currentDir = path.dirname(currentDir)
  }
  errorReporter('Could not find civictheme directory.')
}