Update bash-cache Buildkite plugin to version 2.8.0

This version includes a fix that will remove the need for the
`gem install bundler` workarounds.

Author: mokagio

.buildkite/pipeline.yml

@@ -1,7 +1,7 @@
 # Nodes with values to reuse in the pipeline.
 common_params:
   plugins: &common_plugins
-  - &bash_cache automattic/bash-cache#2.7.0: ~
+  - &bash_cache automattic/bash-cache#2.8.0
   # Common environment values to use with the `env` key.
   env: &common_env
     IMAGE_ID: xcode-13.4.1
@@ -14,9 +14,6 @@ steps:
   - label: "🧪 Build and Test"
     key: "test"
     command: |
-      # See https://github.com/Automattic/bash-cache-buildkite-plugin/issues/16
-      gem install bundler
-
       build_and_test_pod
     env: *common_env
     plugins: *common_plugins
@@ -28,9 +25,6 @@ steps:
   - label: "🔬 Validate Podspec"
     key: "validate"
     command: |
-      # See https://github.com/Automattic/bash-cache-buildkite-plugin/issues/16
-      gem install bundler
-
       validate_podspec
     env: *common_env
     plugins: *common_plugins
@@ -42,9 +36,6 @@ steps:
   - label: "🧹 Lint"
     key: "lint"
     command: |
-      # See https://github.com/Automattic/bash-cache-buildkite-plugin/issues/16
-      gem install bundler
-
       lint_pod
     env: *common_env
     plugins: *common_plugins

.buildkite/publish-pod.sh

@@ -4,9 +4,6 @@ PODSPEC_PATH="WPMediaPicker.podspec"
 SLACK_WEBHOOK=$PODS_SLACK_WEBHOOK
 
 echo "--- :rubygems: Setting up Gems"
-# See https://github.com/Automattic/bash-cache-buildkite-plugin/issues/16
-gem install bundler
-
 install_gems
 
 echo "--- :cocoapods: Publishing Pod to CocoaPods CDN"