ci: publish tarballs for all Angular packages as build artifacts on PR builds (#33321)

Previously, when one wanted to try out the changes from a PR before it
was merged, they had to check out the PR locally and build the Angular
packages themselves (which is time-consuming and wasteful given that the
packages have already been built on CI).

This commit persists all Angular packages on each build as `.tgz` files,
which can be used to install dependencies on an project (supported by
both [npm][1] and [yarn][2]). In addition to individual `.tgz` files for
each package, a `.tgz` file including all packages is also stored, which
can be used to test the packages locally by overwriting the ones in the
`node_modules/` directory of a project.

CircleCI [build artifacts][3] an be used for longer-term storage of the
outputs of a build and are designed to be useful around the time of the
build, which suits our needs.

[1]: https://docs.npmjs.com/cli/install.html
[2]: https://yarnpkg.com/lang/en/docs/cli/add
[3]: https://circleci.com/docs/2.0/artifacts

PR Close #33321

.circleci/config.yml

@@ -559,6 +559,26 @@ jobs:
       # amount of container nodes for this job is controlled by the "parallelism" option.
       - run: ./integration/run_tests.sh ${CIRCLE_NODE_INDEX} ${CIRCLE_NODE_TOTAL}
 
+  # This job creates compressed tarballs (`.tgz` files) for all Angular packages and stores them as
+  # build artifacts. This makes it easy to try out changes from a PR build for testing purposes.
+  # More info CircleCI build artifacts: https://circleci.com/docs/2.0/artifacts
+  #
+  # NOTE: Currently, this job only runs for PR builds. See `publish_snapshot` for non-PR builds.
+  publish_packages_as_artifacts:
+    executor: default-executor
+    environment:
+      NG_PACKAGES_DIR: &ng_packages_dir 'dist/packages-dist'
+      NG_PACKAGES_ARCHIVES_DIR: &ng_packages_archives_dir 'dist/packages-dist-archives'
+    steps:
+      - custom_attach_workspace
+      - init_environment
+      - run:
+          name: Create artifacts
+          command: ./scripts/ci/create-package-archives.sh $CI_PULL_REQUEST $CI_COMMIT $NG_PACKAGES_DIR $NG_PACKAGES_ARCHIVES_DIR
+      - store_artifacts:
+          path: *ng_packages_archives_dir
+          destination: angular
+
   # This job updates the content of repos like github.com/angular/core-builds
   # for every green build on angular/angular.
   publish_snapshot:
@@ -823,6 +843,11 @@ workflows:
       - integration_test:
           requires:
             - build-npm-packages
+      - publish_packages_as_artifacts:
+          # Only run on PR builds. (For non-PR builds, the `publish_snapshot` job.)
+          <<: *only_on_pull_requests
+          requires:
+            - build-npm-packages
       - publish_snapshot:
           # Note: no filters on this job because we want it to run for all upstream branches
           # We'd really like to filter out pull requests here, but not yet available: