Fix the problem of pipelines changes with external PRs [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This MR fixes the problem of rules:changes / only:changes for external pull request pipelines.

Related to #208692 (closed) (also my comments: #208692 (comment 564926644), #208692 (comment 568113733))

The problem is about Ci::Pipeline#modified_paths because we check it while executing rules:changes and only:changes. The external PR is neither a merge_request? nor a branch_updated?. So it returns nil. Then satisfied_by? returns true when modified_paths is nil.

It's behind a feature flag ci_modified_paths_of_external_prs (#330605 (closed)).

Screenshots (strongly suggested)

Example CI Config

build:
  stage: build
  script: echo build

test1:
  stage: test
  script: exit 0
  only:
    refs:
      - external_pull_requests
    changes:
      - .gitlab-ci.yml

test2:
  stage: test
  script: exit 0
  only:
    refs:
      - external_pull_requests
    changes:
      - README.md

test3:
  stage: test
  script: exit 0
  only:
    refs:
      - external_pull_requests

test4:
  stage: test
  script: exit 0
  only:
    changes:
      - .gitlab-ci.yml

test5:
  stage: test
  script: exit 0
  only:
    changes:
      - README.md

Before this MR

Created PR

GitLab CI is seen in the PR

All jobs are run in the pipeline

After this MR

Created PR

GitLab CI is seen in the PR

Jobs with changes:.gitlab-ci.yml are excluded from the pipeline

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team