Fix links in code quality widget

What does this MR do?

Fixes #328292 (closed)

It looks like MR !55708 (merged) removed some path variables that are still being used. That broke links to code in code quality widget.

More specifically:

• variable headBlobPath is being used to generate link to introduced issue in app/assets/javascripts/reports/codequality_report/store/actions.js#L17
• variable baseBlobPath is being used to generate link to fixed issue in app/assets/javascripts/reports/codequality_report/store/actions.js#L18
• variable headPath is being used to check whether head codequality exists in app/assets/javascripts/reports/codequality_report/store/getters.js#L45

My MR simply reintroduces those variables, same as they were before.

Screenshots (before)

• Broken link to code introducing quality regression:

• Broken link to code fixing quality regression:

• Popup about base codequality artifact not being found never shows up, only the generic warning was being shown:

• Vue debug screen showing the wrong link to quality regression

Screenshots (after)

• Fixed link to code introducing quality regression:

• Fixed link to code improving quality:

• Fixed popup about base codequality artifact not being found:

• Vue debug screen showing fixed url:

Does this MR meet the acceptance criteria?

Conformity

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• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
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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.
• This change is backwards compatible across updates, or this does not apply.

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.