Test updater from latest released binary

[travis-hoover-glean]

Summary

• update the e2e updater workflow to download the latest released glean-mdm binary instead of building an old binary from source
• derive the release asset from the runner OS/arch and use it as the old binary in the update test

Testing

• not run (workflow-only change)

Stacked on #78.

[travis-hoover-glean]

Closing in favor of #80, which targets main directly and is independent of the Go migration branch.