chore: Run 'yarn install' when building (#215)

I noticed when showing the development flow to a few engineers, there was one awkward step - you have to `cd site` and run `yarn install`, and then come back to root to `make build` or `./develop.sh`

This just adds `yarn install` as part of `make build` and `./develop.sh` - so whichever one is run first will start work.

develop.sh

@@ -21,6 +21,9 @@ function create_initial_user() {
     http://localhost:3000/api/v2/user
 }
 
+# Run yarn install, to make sure node_modules are ready to go
+yarn --cwd=./site install
+
 # Do initial build - a dev build for coderd.
 # It's OK that we don't build the front-end before - because the front-end
 # assets are handled by the `yarn dev` devserver.