Handle issues with OpenID signing key when generating CI_JOB_JWT

What does this MR do?

With !28063 (merged) we introduced new predefined CI variable - CI_JOB_JWT. This is JWT signed with the OpenID Connect signing Key.

After %12.10 is released we found that some instances may have invalid (or missing) OpenID Connect signing key, which will break generating CI_JOB_JWT and as result break CI too - no jobs can be requested by and assigned to runners.

This MR changes the current behavior to catch and track exceptions and set the CI variable only when the JWT was generated successfully.

Related to #215902 (comment 333363535).

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
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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
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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
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