chore(block-scheduler): pin fallback offset during start-up

[ashwanthgoli]

What this PR does / why we need it:

Pin the fallback offset at the time of scheduler creation to ensure planner uses the same fallback offset on subsequent runs.
Without this, the planner could create jobs that are unaligned if the partition has no commits so far and builders are slow finishing the pending work. On a subsequent planning iteration, it could end up using a different fallback timestamp if there are still no commits.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[github-actions]

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[owen-d]

I think we can handle this more comprehensively later; this is a good stopgap 👍