chromiumchecker: Switch prebuilt Clang url to use .tar.xz

[refi64]

The gzip archives aren't produced anymore:

https://chromium-review.googlesource.com/c/chromium/src/+/5544198

[wjt]

======================================================================
FAIL: test_check (tests.test_chromiumchecker.TestChromiumChecker.test_check)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python3.11/unittest/async_case.py", line 90, in _callTestMethod
    if self._callMaybeAsync(method) is not None:
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/unittest/async_case.py", line 112, in _callMaybeAsync
    return self._asyncioRunner.run(
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/asyncio/base_events.py", line 653, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/home/runner/work/flatpak-external-data-checker/flatpak-external-data-checker/tests/test_chromiumchecker.py", line 51, in test_check
    self.assertRegex(
AssertionError: Regex didn't match: '^[https://commondatastorage.googleapis.com/chromium-browser-clang/Linux_x64/clang-.*\\.tgz$](https://commondatastorage.googleapis.com/chromium-browser-clang/Linux_x64/clang-.*//.tgz$)' not found in 'https://commondatastorage.googleapis.com/chromium-browser-clang/Linux_x64/clang-llvmorg-20-init-1009-g7088a5ed-10.tar.xz'

[refi64]

oops I forgot that test existed (despite me being the one who apparently wrote it), should be fixed now.

[wjt]

Thanks! I don't think the remaining failure is on you :)

[wjt]

Filed #441 for the remaining failing test.