add fallback to default releases.bazel.build url

When BAZELISK_BASE_URL is set and bazelisk fails to download Bazel from that URL, it would either throw an HTTPError or exit with 22. It should instead fallback to the default releases.bazel.build url

Start at the download path that reads BAZELISK_BASE_URL and reproduce the failure when that URL cannot provide Bazel. Trace how the HTTPError or exit 22 is handled, then verify that the default releases.bazel.build URL is tried instead and that a successful fallback completes the download.

Written by the indexing model from the issue text.