Use bash-cache Buildkite plugin version 2.8.0

This version includes a fix that removes the need to run
`gem install bundler` before each step.

.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#2.0.0: ~
+  - &bash_cache automattic/bash-cache#2.8.0
   # Common environment values to use with the `env` key.
   env: &common_env
     IMAGE_ID: xcode-13
@@ -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:2.3.4
-
       build_and_test_pod
     env: *common_env
     plugins: *common_plugins
@@ -27,9 +24,6 @@ steps:
   - label: "🔬 Validate Podspecs"
     key: "validate"
     command: |
-      # See https://github.com/Automattic/bash-cache-buildkite-plugin/issues/16
-      gem install bundler:2.3.4
-
       validate_podspec
     env: *common_env
     plugins: *common_plugins
@@ -40,9 +34,6 @@ steps:
   - label: "🧹 Lint"
     key: "lint"
     command: |
-      # See https://github.com/Automattic/bash-cache-buildkite-plugin/issues/16
-      gem install bundler:2.3.4
-
       lint_pod
     env: *common_env
     plugins: *common_plugins

.buildkite/publish-aztec-pod.sh

@@ -5,9 +5,6 @@ SPECS_REPO="[email protected]:wordpress-mobile/cocoapods-specs.git"
 SLACK_WEBHOOK=$PODS_SLACK_WEBHOOK
 
 echo "--- :rubygems: Setting up Gems"
-# See https://github.com/Automattic/bash-cache-buildkite-plugin/issues/16
-gem install bundler:2.3.4
-
 install_gems
 
 echo "--- :cocoapods: Publishing Pod to CocoaPods CDN"

.buildkite/publish-editor.pod.sh

@@ -5,9 +5,6 @@ SPECS_REPO="[email protected]:wordpress-mobile/cocoapods-specs.git"
 SLACK_WEBHOOK=$PODS_SLACK_WEBHOOK
 
 echo "--- :rubygems: Setting up Gems"
-# See https://github.com/Automattic/bash-cache-buildkite-plugin/issues/16
-gem install bundler:2.3.4
-
 install_gems
 
 echo "--- :cocoapods: Publishing Pod to CocoaPods CDN"