[ci] Pin Chromium version for web tests (#4620)

Switches the web tests from using the version of Chrome installed by the Dockerfile, which is whatever happened to be stable when the image is generated, and thus not hermetic, to a pinned version of Chromium. This uses a slightly modified version of the script that is already used for flutter/packages.

Since Chromium doesn't support mp4 playback, this updates the `video_player` integration tests to use WebM on web instead, to avoid having all the tests fail in CI.

Part of https://github.com/flutter/flutter/issues/84712

script/tool/lib/src/drive_examples_command.dart

@@ -120,7 +120,9 @@ class DriveExamplesCommand extends PackageLoopingCommand {
           '-d',
           'web-server',
           '--web-port=7357',
-          '--browser-name=chrome'
+          '--browser-name=chrome',
+          if (platform.environment.containsKey('CHROME_EXECUTABLE'))
+            '--chrome-binary=${platform.environment['CHROME_EXECUTABLE']}',
         ],
       if (getBoolArg(kPlatformWindows))
         kPlatformWindows: <String>['-d', 'windows'],