build: fix circleci not restoring cache for PRs (#28480)

Currently whenever someone creates a pull request, the NPM dependencies
are downloaded and installed. This is problematic because we have a lot
NPM dependencies with potentially large files that would need to be
downloaded (e.g. the Bazel binaries).

The caches are currently not being restored because we added the
`{Branch}` variable to the CircleCI cache key. Since every PR has a different
branch name (e.g. `devversion/refs/heads/my-banch`), the cache keys would
never match an existing cache key, and the PR would start fresh by
downloading everything.

We can safely remove the `{Branch}` variable from the cache key because
it does not provide any value since the cached files are based on the state
of the `yarn.lock` file and not based on the current branch name. This reduces
our dependency on the slow and sometimes **flaky** Yarn registry. We should
try to depend as few as possible on external services (e.g. see how Saucelabs
flakiness can cause trouble for the caretaker; same applies to flaky Yarn installs)

PR Close #28480

.circleci/config.yml

@@ -17,7 +17,11 @@
 # **NOTE 2**: If you change the version of the docker images, also change the `cache_key` suffix.
 var_1: &default_docker_image circleci/node:10.12
 var_2: &browsers_docker_image circleci/node:10.12-browsers
-var_3: &cache_key v2-angular-{{ .Branch }}-{{ checksum "yarn.lock" }}-node-10.12
+# We don't want to include the current branch name in the cache key because that would prevent
+# PRs from being able to restore the cache since the branch names are always different for PRs.
+# The cache key should only consist of dynamic values that change whenever something in the
+# cache changes. (e.g. Yarn lock file changes --> cached "node_modules" are different).
+var_3: &cache_key v2-angular-{{ checksum "yarn.lock" }}-node-10.12
 
 # Define common ENV vars
 var_4: &define_env_vars