[ci] Reuse .yarn-local-mirror (elastic#198613)

Currently CI is configuring a yarn local mirror that is ignored due to
the repository `.yarnrc` taking precedence.

Instead of configuring this setting, this moves the cached mirror over
to the Kibana directory in line with the repository's configuration.

.buildkite/scripts/bootstrap.sh

@@ -12,12 +12,18 @@ if [[ "${BOOTSTRAP_ALWAYS_FORCE_INSTALL:-}" ]]; then
   BOOTSTRAP_PARAMS+=(--force-install)
 fi
 
-# Use the node_modules that is baked into the agent image, if it exists, as a cache
+# Use the packages that are baked into the agent image, if they exist, as a cache
 # But only for agents not mounting the workspace on a local ssd or in memory
 # It actually ends up being slower to move all of the tiny files between the disks vs extracting archives from the yarn cache
-if [[ -d ~/.kibana/node_modules && "$(pwd)" != *"/local-ssd/"* && "$(pwd)" != "/dev/shm"* ]]; then
-  echo "Using ~/.kibana/node_modules as a starting point"
-  mv ~/.kibana/node_modules ./
+if [[ "$(pwd)" != *"/local-ssd/"* && "$(pwd)" != "/dev/shm"* ]]; then
+  if [[ -d ~/.kibana/node_modules ]]; then
+    echo "Using ~/.kibana/node_modules as a starting point"
+    mv ~/.kibana/node_modules ./
+  fi
+  if [[ -d ~/.kibana/.yarn-local-mirror ]]; then
+    echo "Using ~/.kibana/.yarn-local-mirror as a starting point"
+    mv ~/.kibana/.yarn-local-mirror ./
+  fi
 fi
 
 if ! yarn kbn bootstrap "${BOOTSTRAP_PARAMS[@]}"; then

.buildkite/scripts/common/setup_node.sh

@@ -10,7 +10,6 @@ NODE_VERSION="$(cat "$KIBANA_DIR/.node-version")"
 export NODE_VERSION
 export NODE_DIR="$CACHE_DIR/node/$NODE_VERSION"
 export NODE_BIN_DIR="$NODE_DIR/bin"
-export YARN_OFFLINE_CACHE="$CACHE_DIR/yarn-offline-cache"
 
 ## Install node for whatever the current os/arch are
 hostArch="$(command uname -m)"
@@ -77,8 +76,6 @@ if [[ ! $(which yarn) || $(yarn --version) != "$YARN_VERSION" ]]; then
   npm_install_global yarn "^$YARN_VERSION"
 fi
 
-yarn config set yarn-offline-mirror "$YARN_OFFLINE_CACHE"
-
 YARN_GLOBAL_BIN=$(yarn global bin)
 export YARN_GLOBAL_BIN
 export PATH="$PATH:$YARN_GLOBAL_BIN"