fix(helm): Use variables instead of root context in functions parsed by tpl

[horihel]

What this PR does / why we need it:

Using root context in _helpers.tpl triggers problems in helm, during render and especially during lint (helm/helm#12798)

This change works around the problem by storing the objects in a variable before the chart changes scope. I've added "required" keywords to retain the original behavior (chart fails to render if bucketNames is unset).

Which issue(s) this PR fixes:
Fixes #13284
might also clear up: #13564 (not sure)

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
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• 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

[CLAassistant]

All committers have signed the CLA.

[horihel]

Dear review team. It's now a month in and there was no reaction to this PR. I'm still available to discuss it, but I will pause maintaining it until someone shows up willing to review.