[Jsonnet-linter]: Use `HEAD` commit instead of static `main` branch

[kavirajk]

Describe the bug A clear and concise description of what the bug is. Currently running make jsonnet-linter fail to catch all jsonnet errors in production/promtail-mixin.

Scenario: production/promtail-mixin depends on production/loki-mixin and it always uses the one from main branch.

This is the issue because, let's say production/loki-mixin has some changes that is breaking production/promtail-mixin we unable to catch the issue before merging into main.

This is what happened with one of the loki-mixin PR changes today.

To Reproduce Steps to reproduce the behavior: https://github.com/grafana/loki/pull/6377

Expected behavior Any change in loki-mixin or any mixins under production should be made sure it's not breaking any of the mixins that depends on it before merging into main branch.

Environment: Anywhere.

Screenshots, Promtail config, or terminal output

Proposal

Use HEAD commit in the PR's CI as jsonnet dependency everywhere, rather than using main branch.