Allow skipping before parsing .gitlab-ci.yml

What does this MR do?

Fixes #235855 (closed)

Pushing with the skip option or adding [skip ci] will fail if the .gitlab-ci.yml is invalid or cannot be parsed due to permission issues. This moves the skip stage earlier to prevent an invalid pipeline from spawning.

The risks are very low, in that skipping CI is already indicates opting out of running various pipelines

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 I think this is probably more bug than feature, though I can include it if necessary.
• 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