build(deps): bump actions/checkout from v4 to v6

[jpnurmi]

Hopefully, this helps with the frequent CI failures:

Error: fatal: could not read Username for 'https://github.com/': terminal prompts disabled
  Error: fatal: clone of 'https://github.com/getsentry/crashpad.git' into submodule path '/home/runner/work/sentry-native/sentry-native/external/crashpad' failed
  Failed to clone 'external/crashpad'. Retry scheduled

https://github.com/getsentry/sentry-native/actions/runs/22141839414/job/64007874879