.github: add workflows/bug_snapshot.yaml

This creates a new GitHub workflow that snapshots open bugs in a
compressed format that is convenient to extract for later analysis in
a Python script, using scripts/make_bugs_pickle.py.

The workflow is only run on demand. This allows us to run it during
the weekly release readiness meeting, right after we have finished the
initial bug triage for the week. That ensures that results are as
fresh and relevant as possible.

The resulting file is uploaded to S3; with the goal of enabling later
tracking and analysis.

Getting the analysis scripts themselves merged into Zephyr is left for
future work. This commit just allows us to get the data.

Co-authored-by: Stephanos Ioannidis <root@stephanos.io>
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
Signed-off-by: Stephanos Ioannidis <root@stephanos.io>