Enable MR pipelines in container scanning template

What does this MR do?

In !61850 (merged) we tried to enable MR pipelines for the container scanning template, but the rule was too "loose" and it allowed double pipelines to trigger. Mostly explained here: https://docs.gitlab.com/ee/ci/yaml/README.html#avoid-duplicate-pipelines

We are reverting the change in !62810 (merged)

This MR uses CI_OPEN_MERGE_REQUESTS to allow both MR and Branch pipelines to run without duplication, as discussed at #217668 (comment 502308480)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

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• 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