browse/snapshot_context: Handle mercurial edge case in _get_release

Mercurial branch name for a release has the following pattern:

tags/{release_name}

So handle that edge case when attempting to fetch a release by its name to avoid 404 errors.

See https://archive.softwareheritage.org/browse/release/785a7d39cf591b2b54186f6a06cefab0c78f927d/?origin_url=https://hg.mozilla.org/mozilla-unified/&release=FIREFOX_109_0_RELEASE&snapshot=10ae1d3a1b3806b16931f7ddfa3acf35d20b0ae1