Fix preview builds on CI (#19970)

Preview builds were setup to install correctly on CircleCI prior to
the Yarn v3 upgrade, but that integration broke with that upgrade. The
Yarn and CircleCI configuration has been updated to fix this.

The `.yarnrc.yaml` file has been updated to configure the GitHub
registry but leave it disabled by default. It can be enabled
dynamically using an environment variable. This lets us switch between
registries without updating the file.

The new workflow is documented here: https://github.com/MetaMask/core/pull/1481

.circleci/config.yml

@@ -353,17 +353,15 @@ jobs:
               echo "Not a PR; skipping"
             fi
       - run:
-          name: Setup registry config for using package previews on draft PRs
+          name: Install dependencies
           command: |
             if [[ $IS_DRAFT == 'true' ]]
             then
-              printf '%s\n\n%s' '@metamask:registry=https://npm.pkg.github.com' "//npm.pkg.github.com/:_authToken=${GITHUB_PACKAGE_READ_TOKEN}" > .npmrc
+              # Use GitHub registry on draft PRs, allowing the use of preview builds
+              METAMASK_NPM_REGISTRY=https://npm.pkg.github.com yarn --immutable
             else
-              echo "Not draft; skipping GitHub registry setup"
+              yarn --immutable
             fi
-      - run:
-          name: Install deps
-          command: yarn --immutable
       - save_cache:
           key: dependency-cache-v1-{{ checksum "yarn.lock" }}
           paths:

.yarnrc.yml

@@ -8,6 +8,15 @@ logFilters:
 
 nodeLinker: node-modules
 
+npmRegistries:
+  "https://npm.pkg.github.com":
+    npmAlwaysAuth: true
+    npmAuthToken: "${GITHUB_PACKAGE_READ_TOKEN-}"
+
+npmScopes:
+  metamask:
+    npmRegistryServer: "${METAMASK_NPM_REGISTRY:-https://registry.yarnpkg.com}"
+
 plugins:
   - path: .yarn/plugins/@yarnpkg/plugin-allow-scripts.cjs
     spec: "https://raw.githubusercontent.com/LavaMoat/LavaMoat/main/packages/yarn-plugin-allow-scripts/bundles/@yarnpkg/plugin-allow-scripts.js"