[CI] Run PR against chrome-beta (#192257)

## Summary
This PR's goal is to enable developers (or automation) to test against
chrome-beta with a PR label - to adjust sensitive tests, or to foresee
test breakages.

⚠️ Risk: a PR verified/built with the chrome beta setting, would pass
the PR tests, but might not pass the on-merge suite in the end.

Addresses: elastic/kibana-operations#199
Depends on: elastic/ci-agent-images#907

It highlights the errors visible only on the next versions:
https://buildkite.com/elastic/kibana-pull-request/builds/233373
And it doesn't break with non-beta run:
https://buildkite.com/elastic/kibana-pull-request/builds/233716

.buildkite/pipeline-utils/ci-stats/pick_test_group_run_order.ts

@@ -197,6 +197,32 @@ function getEnabledFtrConfigs(patterns?: string[]) {
   }
 }
 
+/**
+ * Collects environment variables from labels on the PR
+ * TODO: extract this (and other functions from this big file) to a separate module
+ */
+function collectEnvFromLabels() {
+  const LABEL_MAPPING: Record<string, Record<string, string>> = {
+    'ci:use-chrome-beta': {
+      USE_CHROME_BETA: 'true',
+    },
+  };
+
+  const envFromlabels: Record<string, string> = {};
+  if (!process.env.GITHUB_PR_LABELS) {
+    return envFromlabels;
+  } else {
+    const labels = process.env.GITHUB_PR_LABELS.split(',');
+    labels.forEach((label) => {
+      const env = LABEL_MAPPING[label];
+      if (env) {
+        Object.assign(envFromlabels, env);
+      }
+    });
+    return envFromlabels;
+  }
+}
+
 export async function pickTestGroupRunOrder() {
   const bk = new BuildkiteClient();
   const ciStats = new CiStatsClient();
@@ -273,9 +299,10 @@ export async function pickTestGroupRunOrder() {
           .filter(Boolean)
       : ['build'];
 
-  const FTR_EXTRA_ARGS: Record<string, string> = process.env.FTR_EXTRA_ARGS
+  const ftrExtraArgs: Record<string, string> = process.env.FTR_EXTRA_ARGS
     ? { FTR_EXTRA_ARGS: process.env.FTR_EXTRA_ARGS }
     : {};
+  const envFromlabels: Record<string, string> = collectEnvFromLabels();
 
   const { defaultQueue, ftrConfigsByQueue } = getEnabledFtrConfigs(FTR_CONFIG_PATTERNS);
 
@@ -514,7 +541,8 @@ export async function pickTestGroupRunOrder() {
                   agents: expandAgentQueue(queue),
                   env: {
                     FTR_CONFIG_GROUP_KEY: key,
-                    ...FTR_EXTRA_ARGS,
+                    ...ftrExtraArgs,
+                    ...envFromlabels,
                   },
                   retry: {
                     automatic: [

.buildkite/scripts/steps/test/ftr_configs.sh

@@ -57,6 +57,19 @@ while read -r config; do
 
   start=$(date +%s)
 
+  if [[ "${USE_CHROME_BETA:-}" =~ ^(1|true)$ ]]; then
+    echo "USE_CHROME_BETA was set - using google-chrome-beta"
+    export TEST_BROWSER_BINARY_PATH="$(which google-chrome-beta)"
+
+    # download the beta version of chromedriver
+    export CHROMEDRIVER_VERSION=$(curl https://googlechromelabs.github.io/chrome-for-testing/last-known-good-versions.json -s | jq -r '.channels.Beta.version')
+    export DETECT_CHROMEDRIVER_VERSION=false
+    node node_modules/chromedriver/install.js --chromedriver-force-download
+
+    # set annotation on the build
+    buildkite-agent annotate --style info --context chrome-beta "This build uses Google Chrome Beta @ ${CHROMEDRIVER_VERSION}"
+  fi
+
   # prevent non-zero exit code from breaking the loop
   set +e;
   node ./scripts/functional_tests \