test: bump material cache_key version (#34589)

Also add comment that cache_key version should be bumped when switching forks or branches and added a comment from @devversion explaining how the fallback cache key works.

PR Close #34589

.circleci/config.yml

@@ -29,8 +29,8 @@ var_4_win: &cache_key_win_fallback v5-angular-win-node-12.0-
 
 # Cache key for the Material unit tests job. **Note** when updating the SHA in the cache keys,
 # also update the SHA for the "MATERIAL_REPO_COMMIT" environment variable.
-var_5: &material_unit_tests_cache_key v5-angular-material-9e7ba251207df77164d73d66620e619bcbc4d2ad
-var_6: &material_unit_tests_cache_key_fallback v5-angular-material-
+var_5: &material_unit_tests_cache_key v6-angular-material-9e7ba251207df77164d73d66620e619bcbc4d2ad
+var_6: &material_unit_tests_cache_key_fallback v6-angular-material-
 
 # Workspace initially persisted by the `setup` job, and then enhanced by `build-npm-packages` and
 # `build-ivy-npm-packages`.
@@ -752,6 +752,12 @@ jobs:
       - restore_cache:
           keys:
             - *material_unit_tests_cache_key
+            # Whenever the `angular/components` SHA is updated, the cache key will no longer match. The
+            # fallback cache will still match, and CircleCI will restore the most recently cached
+            # repository folder. Without the fallback cache, we'd need to download the repository
+            # from scratch and it would slow down the job. This is because we can't clone the
+            # repository with reduced "--depth", but rather need to clone the whole repository to be
+            # able to support arbitrary SHA's.
             - *material_unit_tests_cache_key_fallback
       - run:
           name: "Fetching Material repository"

.circleci/env.sh

@@ -72,6 +72,8 @@ setPublicVar SAUCE_READY_FILE_TIMEOUT 120
 # completely isolated from angular/angular in order to avoid any bad interactions between
 # their separate build setups.
 setPublicVar MATERIAL_REPO_TMP_DIR "/tmp/material2"
+# **NOTE**: When switching forks or branches, also bump the cache key & fallback cache key versions
+# in the CircleCI "config.yml".
 setPublicVar MATERIAL_REPO_URL "https://github.com/angular/material2.git"
 setPublicVar MATERIAL_REPO_BRANCH "master"
 # **NOTE**: When updating the commit SHA, also update the cache key in the CircleCI "config.yml".