fix: always use LocalOnlyGitRepo source with dry_run command

[imaginaris]

This PR should fix an issue that happens in my project.

We are running danger dry_run on master branch (no PR) on Bitrise CI machines.
After upgrading ruby version from 2.7.6 to 3.1.4 we started to get following error:
/Users/vagrant/.asdf/installs/ruby/3.1.4/lib/ruby/gems/3.1.0/gems/danger-9.3.1/lib/danger/request_sources/bitbucket_cloud_api.rb:125:in fetch_json': Credentials not available. Provide DANGER_BITBUCKETCLOUD_USERNAME, DANGER_BITBUCKETCLOUD_UUID, and DANGER_BITBUCKETCLOUD_PASSWORD as environment variables. (RuntimeError)`

Even though we use Github.com repos and the same Danger version (9.3.1) as before.

The CI.available_ci_sources is a set and local_ci_source method in environment_manager.rb uses a find function to search for the first valid source in that set. A set has undefined order of elements when treated as a list so when there are multiple CI sources valid (in my case Bitrise and LocalOnly), the local_ci_source method (theoretically) can return a different value each time.
Returning Danger::Bitrise value in my case can't work because there is no PR info in the system env when I'm running danger dry_run on no-PR build. (Bitrise class tries to init all RequestSource classes from supported_request_sources one by one).

Long story short, when running danger dry_run only LocalOnlyGitRepo should be used as a CI source.
I did my best in this PR (I'm not a ruby dev 🙂) so any comments are welcome.

[manicmaniac]

@imaginaris
Thanks you for the detailed description. The change looks reasonable to me.
Could you rebase it on master and resolve conflicts?

[imaginaris]

@manicmaniac Done.

[manicmaniac]

@imaginaris Could you resolve the following lint error?

https://github.com/danger/danger/actions/runs/7768462603/job/21188021833?pr=1452

Run bundle exec rubocop
Inspecting 219 files
.........................................................................C.................................................................................................................................................

Offenses:

lib/danger/danger_core/environment_manager.rb:10:[7](https://github.com/danger/danger/actions/runs/7768462603/job/21188021833?pr=1452#step:5:8): C: [Correctable] Layout/EmptyLineAfterGuardClause: Add empty line after guard clause.
      return Danger::LocalOnlyGitRepo if LocalOnlyGitRepo.validates_as_ci? env
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

21[9](https://github.com/danger/danger/actions/runs/7768462603/job/21188021833?pr=1452#step:5:10) files inspected, 1 offense detected, 1 offense autocorrectable

[imaginaris]

@manicmaniac Fixed. The CI job should pass now.