chore: add script to install yarn dependencies

* Use frozen lockfile in build for reproducible builds
* Do not install optional dependencies
* Suppress interactive prompts in build

develop.sh

@@ -22,7 +22,7 @@ function create_initial_user() {
 }
 
 # Run yarn install, to make sure node_modules are ready to go
-yarn --cwd=./site install
+"$PROJECT_ROOT/scripts/yarn_install.sh"
 
 # Do initial build - a dev build for coderd.
 # It's OK that we don't build the front-end before - because the front-end