CI: Fix upload of screenshots in gitlab:ui:visual

What does this MR do?

Before this commit uploading of screenshots failed with

WARNING: tests/__image_snapshots__/: no matching files
ERROR: No files to upload
ERROR: Job failed: exit code 1

See https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/203024045

After this commit it works again

Uploading artifacts...
gitlab-ui/tests/__image_snapshots__/: found 90 matching files
Uploading artifacts to coordinator... ok

See https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/203252304

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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