Fix db_tasks script

What does this MR do and why?

With !110361 (diffs) we converted scripts/db_tasks from bash to ruby, and we should use # instead of $ when interpolating variable in string.

Currently jobs using this script fail, for example https://gitlab.com/gitlab-org/gitlab/-/jobs/3699879184.

How to set up and validate locally

1. On master

   $ RAILS_ENV=development scripts/db_tasks db:migrate:status
   Running: `bundle exec rake ${task}:main`
   rake aborted!             
   Don't know how to build task '${task}:main' (See the list of available tasks with `rake --tasks`)
   /Users/krasio/.asdf/installs/ruby/3.0.5/bin/bundle:25:in `load'
   /Users/krasio/.asdf/installs/ruby/3.0.5/bin/bundle:25:in `<main>'
   (See full trace by running task with --trace)

2. On this branch

   $ RAILS_ENV=development scripts/db_tasks db:migrate:status
   Running: `bundle exec rake db:migrate:status:main`
   
   database: gitlabhq_development
   
    Status   Migration ID    Migration Name
   --------------------------------------------------
      up     20210301200959  ********** NO FILE **********
      up     20210302025305  ********** NO FILE **********
      up     20210302074524  ********** NO FILE **********
      ...

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