[Dependency Cache] Fix Dependency Cache (#138)

* Gradle: Fix dependency cache

As part of #135 and this last 1dda7bd
commit I incorrectly removed the 'DEP_CACHE_FOLDER_NAME' variable.
Without this variable set, placing Gradle dependency cache is not
working as expected due to the fact that this 'DEP_CACHE_FOLDER_NAME'
variable is now unbound. And this makes this whole dependency cache
solution not working at all.

* Release: Update `README` and `CHANGELOG` with 3.9.0 release info (#135)

FYI: Lint is now failing otherwise: https://buildkite.com/automattic/
ci-toolkit-buildkite-plugin/builds/
894#01946971-8ca9-4e6b-8024-da9e25f85121

PS: I wasn't aware about this process and thus missed updating both, the
'README' and 'CHANGELOG' with 3.9.0 release info after merging #135 by
creating a separate 'release/3.9.0' branch and PR for release purposes.
Instead, I just went and drafted a new release directly from GitHub and
the project's releases' page.

Example Release Process Related PR: https://github.com/Automattic/
a8c-ci-toolkit-buildkite-plugin/pull/137

* Release: Add 'CHANGELOG' entry

CHANGELOG.md

@@ -42,12 +42,19 @@ _None._
 
 ### Bug Fixes
 
+- `Dependency Cache`: Fix Dependency Cache [#138]
 - `run_swiftlint`: Error gracefully when `swiftlint_version` is missing in the `.swiftlint.yml` file [#139]
 
 ### Internal Changes
 
 _None._
 
+## 3.9.0
+
+### New Features
+
+- `Dependency Cache`: Dependency Cache on CI per Project `[without GRADLE_RO_DEP_CACHE]` [#135]
+
 ## 3.8.0
 
 ### New Features

bin/restore_gradle_dependency_cache

@@ -9,6 +9,7 @@ echo "Restoring Gradle dependency cache..."
 GRADLE_DEP_CACHE="$GRADLE_HOME/dependency-cache"
 
 DEP_CACHE_BASE_FOLDER=$(dirname "$GRADLE_DEP_CACHE")
+DEP_CACHE_FOLDER_NAME=$(basename "$GRADLE_DEP_CACHE")
 
 # `save_cache` & `restore_cache` scripts only work if they are called from the same directory
 pushd "$DEP_CACHE_BASE_FOLDER"