Remove dependency on the database from the shellcheck templates lint

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The following discussion from !80435 (merged) should be addressed:

• @kwiebers started a discussion: (+4 comments)

  Is shellcheck a static analysis linter for the template files? If so, is there a need for Postgres or Rails cache?

@godfat-gitlab:

This feels really heavy because even setup-test-env doesn't need to use the database, and a lot of our rake tasks don't need database either.

I wonder if it's possible to use Gitlab::Ci::YamlProcessor without booting the whole Rails application? In theory it shouldn't really need to connect to a database? I see if this takes too much effort to do, we can take that to next iteration.

@bwill:

I tried to get this to work without the DB, but I can't seem to find a way to get the rails environment to load without the DB: https://gitlab.com/gitlab-org/gitlab/-/jobs/2129593347

Reading https://stackoverflow.com/questions/821251/how-to-configure-ruby-on-rails-with-no-database, I think we would probably have to add a new environment in config/environments/ with no ActiveRecord, or maybe even control it with an environment variable in config/application.rb. I would rather not touch those files as part of this MR, so I will leave that to a later iteration.