Gradle: Add v2 suffix on the gradle dependency cache key

This is done in order to differentiate between the 'GRADLE_RO_DEP_CACHE'
and the 'GRADLE_DEP_CACHE' caches, and thus be able to test both caching
strategies in-parallel, without one overwriting the other on save, or
one reusing the other on restore.

bin/restore_gradle_dependency_cache

@@ -1,7 +1,7 @@
 #!/bin/bash -eu
 
 # The key is shared with `bin/save_gradle_dependency_cache`
-GRADLE_DEPENDENCY_CACHE_KEY="${BUILDKITE_PIPELINE_SLUG}_GRADLE_DEPENDENCY_CACHE"
+GRADLE_DEPENDENCY_CACHE_KEY="${BUILDKITE_PIPELINE_SLUG}_GRADLE_DEPENDENCY_CACHE_V2"
 
 echo "Restoring Gradle dependency cache..."
 

bin/save_gradle_dependency_cache

@@ -1,7 +1,7 @@
 #!/bin/bash -eu
 
 # The key is shared with `bin/restore_gradle_dependency_cache`
-GRADLE_DEPENDENCY_CACHE_KEY="${BUILDKITE_PIPELINE_SLUG}_GRADLE_DEPENDENCY_CACHE"
+GRADLE_DEPENDENCY_CACHE_KEY="${BUILDKITE_PIPELINE_SLUG}_GRADLE_DEPENDENCY_CACHE_V2"
 
 echo "Saving Gradle dependency cache..."