ci: pin ChromeDriver to a version compatible with docker image's Chrome (#28494)

By default, `webdriver-manager update` will download the latest
ChromeDriver version, which might not be compatible with the Chrome
version included in the [docker image used on CI], causing CI failures.
Previously, we used to pin the ChromeDriver version on CI in
[ngcontainer's Dockerfile][2]. This was accidentally broken in #26691,
while moving from ngcontainer to default CircleCI docker images.

This commit fixes the issue by pinning ChromeDriver to a known
compatible version.

[1]: bfd48d156d/.circleci/config.yml (L16)
[2]: bfd48d156d/tools/ngcontainer/Dockerfile (L63)

PR Close #28494

integration/platform-server/package.json

@@ -34,7 +34,7 @@
     "webpack": "^2.2.1"
   },
   "scripts": {
-    "postinstall": "webdriver-manager update --gecko false --standalone false $CHROMEDRIVER_VERSION_ARG",
+    "postinstall": "webdriver-manager update --gecko false --standalone false $CI_CHROMEDRIVER_VERSION_ARG",
     "build": "./build.sh",
     "test": "npm run build && concurrently \"npm run serve\" \"npm run protractor\" --kill-others --success first",
     "serve": "node built/server-bundle.js",