ios: Ignore failures to obtain runtime in CIPD in validate_local_ios_runtime()

Non-Googlers do not have access to infra_internal in CIPD, which meant
that run.py would fail with an error similar to this:

    [05:08:33:DEBUG] cipd describe cmd ['cipd', 'describe', 'infra_internal/ios/xcode/ios_runtime_dmg', '-version', '16c5032a'] returned nothing
    [05:08:34:DEBUG] cipd describe cmd ['cipd', 'describe', 'infra_internal/ios/xcode/ios_runtime_dmg', '-version', 'ios-18-2'] returned nothing
    Traceback (most recent call last):
      File "ios/build/bots/scripts/run.py", line 115, in run
        install_success, is_legacy_xcode = xcode.install_xcode(
      File "/Users/rakuco/src/chromium-ios/src/ios/build/bots/scripts/xcode_util.py", line 587, in install_xcode
        validate_local_ios_runtime(xcode_build_version, ios_version)
      File "/Users/rakuco/src/chromium-ios/src/ios/build/bots/scripts/xcode_util.py", line 568, in validate_local_ios_runtime
        local_runtime = iossim_util.get_simulator_runtime_info_by_build(runtime_build)
      File "/Users/rakuco/src/chromium-ios/src/ios/build/bots/scripts/iossim_util.py", line 345, in get_simulator_runtime_info_by_build
        if runtime['build'].lower() == runtime_build.lower():
    AttributeError: 'NoneType' object has no attribute 'lower'

Even when the appropriate platform is installed locally.

Change-Id: I218a30483f773490cd5afbdc5713e918d7bc5cb1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6181162
Auto-Submit: Raphael Kubo da Costa <kubo@igalia.com>
Commit-Queue: Raphael Kubo da Costa <kubo@igalia.com>
Reviewed-by: Yue She <yueshe@google.com>
Cr-Commit-Position: refs/heads/main@{#1409077}