[ci] Fix yarn-offline-mirror (elastic#196224)

Currently CI is configuring a yarn offline mirror outside of the Kibana
directory, with the intention of caching assets during image build. This
configuration is ignored due to .yarnrc taking precedence, resulting in
the offline mirror being setup in the local Kibana installation. On CI
start, a fresh checkout of the repository is made and the cache
directory is empty.

Instead of setting a user level configuration this modifies .yarnrc with
the intended directory.

.buildkite/scripts/common/setup_node.sh

@@ -77,7 +77,8 @@ if [[ ! $(which yarn) || $(yarn --version) != "$YARN_VERSION" ]]; then
   npm_install_global yarn "^$YARN_VERSION"
 fi
 
-yarn config set yarn-offline-mirror "$YARN_OFFLINE_CACHE"
+grep -q 'yarn-offline-mirror ".yarn-local-mirror"' .yarnrc
+sed -i "s#.yarn-local-mirror#$YARN_OFFLINE_CACHE#" .yarnrc
 
 YARN_GLOBAL_BIN=$(yarn global bin)
 export YARN_GLOBAL_BIN

.buildkite/scripts/common/util.sh

@@ -33,7 +33,7 @@ check_for_changed_files() {
 
   SHOULD_AUTO_COMMIT_CHANGES="${2:-}"
   CUSTOM_FIX_MESSAGE="${3:-}"
-  GIT_CHANGES="$(git status --porcelain -- . ':!:.bazelrc' ':!:config/node.options' ':!config/kibana.yml')"
+  GIT_CHANGES="$(git status --porcelain -- . ':!:.bazelrc' ':!:.yarnrc' ':!:config/node.options' ':!config/kibana.yml')"
 
   if [ "$GIT_CHANGES" ]; then
     if ! is_auto_commit_disabled && [[ "$SHOULD_AUTO_COMMIT_CHANGES" == "true" && "${BUILDKITE_PULL_REQUEST:-false}" != "false" ]]; then
@@ -56,7 +56,7 @@ check_for_changed_files() {
       git config --global user.name kibanamachine
       git config --global user.email '[email protected]'
       gh pr checkout "${BUILDKITE_PULL_REQUEST}"
-      git add -A -- . ':!.bazelrc' ':!WORKSPACE.bazel' ':!config/node.options' ':!config/kibana.yml'
+      git add -A -- . ':!.bazelrc' ':!.yarnrc' ':!WORKSPACE.bazel' ':!config/node.options' ':!config/kibana.yml'
 
       git commit -m "$NEW_COMMIT_MESSAGE"
       git push