Use dependabot-common gem if available

[kbukum1]

What are you trying to accomplish?

We should try to load the common/lib path and fallback to the gem. This should cover running in the dry-run, api and now dependabot-omnibus gem.

The files could not be located when running in the dependabot-omnibus gem.

An error occurred while loading ./spec/actions/activate_account_spec.rb.
Failure/Error: require File.expand_path("../config/environment", __dir__)

Zeitwerk::Error:
  the root directory /home/dependabot/.local/share/gem/ruby/gems/common/lib does not exist
# /home/dependabot/.local/share/gem/ruby/gems/zeitwerk-2.7.1/lib/zeitwerk/loader/config.rb:125:in `push_dir'
# /home/dependabot/.local/share/gem/ruby/gems/dependabot-bun-0.296.2/lib/dependabot/bun.rb:9:in `<main>'
# /home/dependabot/.local/share/gem/ruby/gems/bootsnap-1.18.4/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:30:in `require'
# /home/dependabot/.local/share/gem/ruby/gems/dependabot-bun-0.296.2/lib/dependabot/javascript.rb:4:in `<main>'
# /home/dependabot/.local/share/gem/ruby/gems/bootsnap-1.18.4/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:30:in `require'
# /home/dependabot/.local/share/gem/ruby/gems/dependabot-omnibus-0.296.2/lib/dependabot/omnibus.rb:23:in `<main>'
# ./config/application.rb:47:in `<top (required)>'
# ./config/environment.rb:4:in `require_relative'
# ./config/environment.rb:4:in `<top (required)>'
# ./spec/rails_helper.rb:4:in `<top (required)>'
# ./spec/actions/activate_account_spec.rb:3:in `<top (required)>'
# ------------------
# --- Caused by: ---
# LoadError:
#   cannot load such file -- dependabot-omnibus
#   ./config/application.rb:47:in `<top (required)>'

An error occurred while loading ./spec/actions/azure_blob_storage_spec.rb.
Failure/Error: require File.expand_path("../config/environment", __dir__)

Anything you want to highlight for special attention from reviewers?

Ideally we could test building the dependabot-omnibus gem without going through the whole release process.

How will you know you've accomplished your goal?

dependabot-omnibus should build successfully.

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.