Add yarn install before build

[alexkn]

Without yarn install you get this:

~/rhasspy$ yarn build
yarn run v1.21.1
$ vue-cli-service build
/bin/sh: 1: vue-cli-service: not found
error Command failed with exit code 127.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

[synesthesiam]

Good catch, thank you.