ci: prevent multiple builds

* Updates the GH Actions workflow to use very specific
  commands for each step that we want to invoke. This
  provides clarity as to what is happening in CI.
* Additionally, these specific commands bypass Nx,
  which was not caching the component build step and
  was running multiple times, which caused broken asset
  paths in the docs site.

Author: pfulton

.github/workflows/pr-build-and-deploy.yml

@@ -19,8 +19,16 @@ jobs:
           node-version: 16
       - name: Install dependencies
         run: yarn install
-      - name: Build the site and deploy to Netlify
-        run: yarn prepare:site
+      - name: Clean everything
+        run: yarn clean
+      - name: Build components
+        run: yarn build:components
+      - name: Build docs site
+        run: yarn build:site
+      - name: Build Storybook (preview)
+        run: yarn workspace @spectrum-css/preview build
+      - name: Move Storybook build to dist
+        run: yarn postbuild:all
       - name: Deploy to Netlify
         uses: nwtgck/actions-netlify@v2
         with: